The very first thing to check is.......<drum-roll>......warranty. Do you have warranty? If so take it back for coverage.
I would not suspect a low power problem, but easy enough to monitor with a volt meter at the outlet while playing. The voltage should be around 110 to 120 VAC, and remain quite steady. The voltage would have to sag quite a bit for it to begin to be an issue. It is possible, but highly unlikely. I would consider improper grounding as a possible issue though. Are you using a three-prong outlet with a definite separate ground? Death to those who defeat the ground! (<--- battle cry)
Did the tech take the amp apart to make sure that the solder joints are good? If not, that should be done after ground/power issues are eliminated as a possible problem. An intermittent problem usually indicates a poor connection. Typically, solder joints are the first suspect in a modern amp if there is an intermittent connection.
Those are the things that I would check; monitor the outlet voltage, proper ground, solder joints. Anything beyond that could get hairy, and solid state amps aren't fun/popular to work on.
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---
Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.