Several years back, I got a brand new Crate GFX120.  It's one of the older ones, not the ones with the red/black scheme, but blue/black color scheme.  It shot sparks from a short last year.  It is a 1x12 120w combo.  I recently decided to fix it up. Replaced the fuse (3A 250V Slow Blow), and the thing powers up and plays fine. However, after maybe 10 minutes of playing, the sound starts to become extremely inconsistent. For instance, I will play a chord on the guitar, and the sound fades to a very low and quiet level. It will keep fading in and out according to the output volume of my guitar (I assume). If I cut the volume on my guitar (turn down the knob all the way), then the hum from the distortion channel from the amp slowly comes back and stays consistent. This leads me to believe my guitar is "overloading" something in the amp. A visual inspection leads me to believe nothing is wrong with the amp (no blown caps or ICs etc.).  It is quite hard to explain.

Any clues? I can take pics of the amp, videos, sound clips etc.

I've confirmed it's not the speaker of the amp.  My gut tells me it's in the poweramp stage.