any help would be appreciated.

built a trinity TMB model 18 watter last year and it is just divine. however lately it will seem to work fine for about 15 mintues and then slowly fizzle out until it makes no sound at all. I've checked the cables and guitar, and they seem to be in working order. also, while in the high gain input with the boost on on the TMB channel, it makes this loud rattling sound sometimes when I play. any thoughts..?
Idk why, maybe a contact in the tube is melting and loosing contact, then rejoining after the tubes cooled...I had the same thing happen with my head lights on my truck, the light would randomly go out, but if you hit it, the filament would weld itself back together and work, but break the next time turned the light back on.

It's a long shot, but it could be a possibility.

Other than that, maybe a cold solder joint within the amp.