I own a Peavey 5150 signature that started to show some volume and sound problems : After something like 20 minutes playing (it's getting kinda regular) the volume drops instantly and the sound is incredibly dark and far. If I plug a patch cable in the loop the problem disappear as fast as it appeared... until it comes back. Pedals, patch or nothing in the loop doesn't really matter as the problem always occur at some point. It happens as well that after a volume drop and getting back to normal the amp sounds thin and "high focused" (it happen really few times). There are no signs that predict when it will happen, it's basically : riffs - riffs - riffs - better riffs - enjoyement - happiness - riffs - volume drop.
I'd like to identify the problem : power tube dead? preamp tube? phase tube? me playing too bad the amp refuse working correctly
? or something else ? Thanks 
William