starcitysongs

I have a friend who brought me two different Marshall heads with the same problem. I'm the resident tinkerer in this neck of the woods and have built a couple amps from kits, done some repairs and the such but have not had much experience with circuit board based amps. The two heads are a TSL 100 dual reverb and a JCM 900 50 watt. Both play fine for a few minutes but then pop or crackle and drop down to maybe 1/8th volume if that much. In other words practically inaudible. I checked the power tubes first and replaced them with known good tubes I had at my shop and the same thing happened. I ran a line out of the effects loop to another power amp and had no problems. Even when the problem was happening the send was giving proper signal so the problem is isolated to the power section. I also read in another post that the 16 ohm jack is notorious for burning out and causing problems. I checked them both and saw no signs of damage but went ahead and replaced one of them just in case it was hidden damage but that did not change anything. I checked the boards and saw no visual signs of arcing or burning and all the components around the power tubes test accurately with what is on the schematic. Any thoughts, ideas?
-thanks
Aaron