Help: Sterling sound fades to lame

Just acquired 2001 Sterling 4H. Plug it in and all is fine for about 10-15 seconds then the sound fades a bit and becomes anemic. Unplug for a while, try again, same result. All controls operate normally but it sounds compressed with no balls and a bit less gain than normal. Battery replaced, pots, switch and jack connections cleaned. Visual inspection reveals nothing out of the ordinary. Tested with multiple amps and cables.

Any ideas before I start shotgunning components? Is there a component layout available for the PCB or is that super-secret EBMM stuff?

Thanks for any help!