All, I posted this at DIYMA, and didn't get too may helpful responses, so figured I'd ask the Hive Mind. Back in the '90s, I used to set amp gains by turning the level pot all the way up and then ...
Is there any reason the wire would move intermittently? That combined with a loose connection certainly can be the reason. Other stuff, power-generation problem in the amp. Like a bad solder-point, ...