Triple Rectifier problem

Hello all, I have a 2003 Triple Rectifier That has suddenly been giving me a problem. It will play perfectly fine for an hour or so and suddenly my volume will drop by half and the first 3 power tubes (from left to right) will be glowing red hot. I immediately shut the amp off the first time this happened and ordered new tubes from Mesa. Shipping is taking a while so I decided to power the amp on to jam a little bit and everything was fine...For about another hour then the same thing happened again so I pulled the plug. Anyhow, to my question: If a power tube or tube is responsible for this, is it normal for the problem to be intermittent? I'm no tube expert, I just assumed when a tube had run it's course it would be done functioning and that would be the end of it. I'm just wondering if there may be something else going on here. Any input would be very much appreciated, Thanks.