Hi folks. There aren't any amp techs in my town and my Orange Th30 head is acting up. First thing I do when I get to practice is turn it on to warm up while we set up. For the first hour and a half of band practice, it cuts in and out from full power to barely above a whisper at random(I've been using someone else's Marshall head as a backup). About an hour and a half of rocking out it starts to hold steady with out cutting out. Does anyone have any ideas where I can start hunting down the issue?

I bought it off reverb last year. It was shipped with the tubes removed and safely wrapped. I plugged them in and went to town. That's the only history I have with it.