Hey;
Does the sound fade out, go away, and then come back? Again, and again until it happens all the time? It happened to me about 6 months ago. Still under warranty so it went back. Has been working great for 6 months. Happened again last week. I got an extended warranty, but I don't want to not have an amp for a month.
I read somewhere that these things eat tubes, so it may be that. Let me know if you figured your issue out.