Do you have excessive fret wear? that can be a detuner.

If you have the bridge floating, how high is the back of the bridge off the top of the guitar?

Fender recommend 1/8" and that has always worked for me. I also have a superb local tech hand cut and fit new tusque (Graphtec) nuts for me, in all of my guitars that have trems I use. A good hand made nut made of slippy material will always improve tone and tuning stability compared to a built to budget nut.

Think about how you attach strings too. You can go mad and fit locking Sperzels (or their eqivalent) for a quick fix, but it's entirely possible to 'lock' the string to a standard good quality tuner. I can tell you how if you need to know or don't already know..

In contrast to some others, I find a floating trem to be more stable. After some serious waggle action, a quick down n up wobble on the arm will clear any fiction sticking and you're good to go, back in tune again.

One other thing, certainly get those missing bridge screws back in - thats a high stress point and you're lucky not to have sheered off the 2 remaining ones, but also check the four neck bolts are nice and tight. I had one guitar that I could not get stable - missed the obvious fact that the neck was moving.. d'oh!

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Fender Highway & Classic 60s Strats, Fender Toronado, Telecaster, Gretsch Projet, Charvel 3, PRS SE Soapbar II & Custom 24, Burns Batwing and many others!

Evidently, you now need to see a Qualified Tech !!

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"C'est parti mon Kiki "

I prefer to have 5 springs for my Strat's trem and I never set it up to float. I never really cared a lot about bending notes up with a 6-screw vintage Strat system personally.

My Strat hardly ever goes out of tune and I can do dive bombs with it if I really feel so daring (not that I really ever do). Sure, every now and then a string might go flat, but no big deal. You learn to live with it and accept it as charm.

Put those four missing screws back in, adjust the trem claw so that the bridge plate is flush to the body and throw 5 springs in there and tell us if that helps. Make sure everything is buckled down tight but not over done.

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2006 Fender AVRI 70s Stratocaster: Natural Ash | Sovtek MIG 60 & A No Name Cabinet | Pedals & Junk

I wouldn't dream of criticising Phoenix Caster in his above post, but it's certainly a common response to Strat tuning problems - ie locking the bridge down flat to the guitar's top. And it works, and probably gives you more tone and sustain into the bargain, and still lets you pitch down.

But.. Part of the charm of Fender's design, is that you can pitch bend up as well as down. That wobble that puts the note centre but, allows a wider vibrato either side of the target note. A much more natural vibrato. That sound is not unique (a Bigsby will do that as well) but is for me, a major charm of the "Fender Sound".

I'd argue that the Fender vibato does it better than the Bigsby, being less (over) engineered and difficult to string and generally get on with. Infact, the only thing I'd put a Bigsby on, is EBay.. All in all, the Strat trem system remains a superb piece of design - seems a shame not to use it to it's full creative potential. Well worth working through the tuning problems to get it working right.

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Fender Highway & Classic 60s Strats, Fender Toronado, Telecaster, Gretsch Projet, Charvel 3, PRS SE Soapbar II & Custom 24, Burns Batwing and many others!

Certainly, the tremelo on a Strat is a major part of it's success, but for folks like me, who don't use it, (and there are many) adding springs, locking down the bridge, etc. is a viable means of avoiding tuning issues. I don't even keep the bar mounted, I keep that in my toolbox.

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'08 Fender Am. Std. Jazz bass, maple fretboard, charcoal frost metallic finish
'10 Squier VM Jazz bass, maple fretboard, natural finish
Ampeg BA115
'09 MIM standard Strat, maple fretboard, sunburst finish
Fender Super Champ XD

Ah, it's a preference for me. I had my old MIM Strat set up to float and I found that I was always bending notes up in pitch when I would rest my hand on the bridge to palm mute notes. I'm a heavy handed player: my picking hand is brutal and my fretting hand has the attitude of SRV but the ability of Jack White (not saying White's a bad guitar player, in fact I really like what he does).

I didn't have any tuning problems with the MIM when it was set to float, but I just couldn't stand to hear a note go up in pitch when I didn't want it to; it made things twice as sloppy and I really didn't like it.

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2006 Fender AVRI 70s Stratocaster: Natural Ash | Sovtek MIG 60 & A No Name Cabinet | Pedals & Junk

3 of my 4 Strats have vintage trems with 3 springs and they never go out of tune.The whole secret is to spend more time fine tuning your set up.I deck all my bridges and pre stretch all my strings when I first put them on. To pre stretch I change one string at a time,bring the string up to pitch bend it as far as possible and crank the whammy and bring it up to pitch again and so on until it stays at the same pitch then I move to the next one. That system has never let me down,I can bend my B string on the 12th fret up to an E and it still stays in tune.The number of winds on your string post also plays a big role,I never go over 3 wraps and usually have only 2.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

Agreed on pre-stretching new strings. I didn't do that well enough on my Strat last night. Good thing she's a forgiving girl.

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2006 Fender AVRI 70s Stratocaster: Natural Ash | Sovtek MIG 60 & A No Name Cabinet | Pedals & Junk

Finally the Fruddua method doesn't work for bendings.

I returned to the Carl Verheyen method and it works fine, balancing intune after bendings and tremolo light use... Enough for me.

thanks.

Saludos desde España.

Eric Johnson has some really vintage Strats with the vintage trem and his stay in tune even dumping the arm. Building your Strat type guitar out of parts is the main problem. There are very few guitar techs that know how to work on a Strat to make it stay in tune well but it can be done. Every string contact point on the bridge and string trees and nut has to be smooth and lubricated to prevent binding. You would have to take the guitar to an exceptional guitar tech to work on it. the new American Standard and American Deluxe guitars stay in tune very well right from the factory. You might have to buy locking tuners like Sperzels and then you don't have to use string trees with those types of hieght graduated tuners. the nut has to be cut properly and lubed as well as the contact points on the bridge including the holes where the strings contact the bridge plate need to be smooth and bevelled.