Hey guys im a nub here and to the solid state world. I have a Crate gx-212 and i like it, it perfectly fits into the trunk of my rx8 and is perfect for practice (120w) so I desperately want to fix it. Getting another practice amp would be a waste i think. Heres whats up:

-After playing for about 1/2 an hour at any setting above 5 (50%) the gain level drops to the point where the lead channel sounds like an overdriven clean channel, and then the clean channel can barely be heard(though it remains clean sounding.)
-This happens no matter which guitar or cable I use, AND it happens even when I use an extension cab.
-Nothing inside the amp is discolored or "burnt" looking.
-http://www.ssguitar.com/index.php?action=dlattach;topic=1075.0;attach=612-schematic

I found the schematic for this amp in the schematic section. I have built tube amplifiers before and i know how to use a dmm so just let me have it straight don't dull it down for me. I don't have an oscilloscope but I can get one. What should i check?

also if I fix it and don't fail I have plenty of other broken solid state crap that I will be able to fix, so I want to learn, not just fix and forget.