It's not so much the warped, it's conductivity in the circuit boards. And every summer in the US, well heck, anywhere, it seems humidity makes it worse. A MUCH DISCUSSED TOPIC on these pages! The boards are black, because they're loaded with carbon black to make them look that way. And carbon - - - is conductive. Not as much as metals, but enough to raise hob with old Fenders, and even brand new kit builds that use this material. Ask Randall...
Enzo sez, shoot hot air from a hair dryer or heat gun onto the board, might help out. Sometimes yes, sometimes not so much.
What I do, is move the sensitive portions of the audio circuit onto tie strips. Looks a little messy but solves the problem at a reasonable price. If the amp's owner has deep pockets & a willingness to spend on a permanent solution, you can rip out the offending circus boards & replace with a point to point board made of epoxy-fiberglass or some other material that won't conduct at random. One source is Doug Hoffman in North Carolina - he was one of the first to offer these back in the early 90's. Probably a dozen or more other sources across the USA and I'm sure in other parts of the world. Doug might even point out who - you oughta contact him if this solution is in the amp's future.