Hello everyone, I'm a newbie here. I have an Archon 25 combo amp that drops volume on the lead channel after 15 minutes or so. Then a few minutes later, the volume will pop back up. I tested all the tubes on a B&K 707, and they all pass. No shorts or grid emissions, and good life expectancy. With the chassis on the bench, I have been tapping various locations on the board while playing, but that doesn't give me a clue to the location of a possible loose connection. Does anyone have an idea if there are any particular components I should suspect? Thanks in advance!