I have a SF Drip Edge Dual Showman head that sputters. I took the chassis out and it plays okay until I tap the chassis and then the volume drops by about half, and then goes back to full power randomly. I'm about to say that physical jarring causes the power to drop and come back. I'm ready to fire up the soldering iron and touch every point. Whoever had the amp before me had it serviced as it has new(er) filter caps: F&T and some no-namers for the high voltage ones under the pan. The bypass caps have all been replaced too. Please help. Thanks.