I am working to fix the buzz/intermittent sound on a Pbass.

When I plug in the cord is very touchy. If I move it up a bit the bass
will play. If I don't touch the cord, then it tends to just not make any
sound at all...nor can I tune it through a tuner.

I have the pickguard off and it seems that all wiring is soldered in place.
I added copper shielding to the inside of the cavity.

One interesting thing...I left the pickguard unscrewed and strung up the
guitar. When I have the cord plugged into the jack and the pickguard down,
the problem occurs. However, when I lift the pickguard up a bit...lifting the
input jack out of the cavity a bit...the guitar tends to make sound.

Any ideas?

It sounds like a grounding issue.

you said you checked all wiring, but does that include the connections on the input jack? the first thing i'd check would be that. loud buzzing would mean ground wire needs soldering, whereas no sound at all means hot wire needs soldering.

unfortunately, i cant picture how everything is laid out inside a Pbass (wiring and components). is it the same as a strat?
possibly, one of the wires may be severed near the connection to the jack. over time, wear and tear can create weak spots or kinks in wire, causing them to just snap apart at that spot, leaving you with no connection. perhaps when you place your pickguard a certain way it either creates contact, or pulls the soldered bit of wire (attached to jack) away from the rest of the wire. its happened to me before.

try to examine a bit closer, from your description, i'd bet this is the issue.

Yep, I just lost the ground on my 72 Strat. It's the first time I had to repair it in 37 years. Glad that's all it is.