Ok, so I have a marshall JCM 2000 dsl 100 all tube head.

It's great. except for one thing.

Lately, it's been dying on me whenever i crank it very high. At all. like, if i go past 5 on my clean channel the amp just sort of "gives up" and the volume nearly dies completely for a few seconds. I hate it. Any suggestions?
i just got my tubes checked recently...i certainly hope that's not the problem. Winged C's are expensive.
they took the tubes out and checked their emission rating, among other things. This was about 3 weeks ago. They were all pretty high, except for one, which was replaced promptly.