By "a while" I mean 2-3 hrs. Volume is strong and then it just fades away. I've tested all tubes on my BK tube tester and all but V1 tested strong. I replaced V1, but the problem is still there. Banging on the amp doesn't seem to help. Flipping the stand-by switch on and off doesn't, either. What does work is turning both the power switch and the standby off and then back on in 15-20 seconds, but even this doesn't always work and the amp loses volume again within a minute or two. The amp is an early 90's RI. Could I be looking at bad caps, or is this a cold/bad solder joint issue?