help me fix my mkiii

[BrANjiSpAd]

I can play the amp a few minutes and then all the volume drops. I can switch to stand by a minute and fire it back up and I'm ok for about a minute...Then its back to cutting out.

Its never given any problems before EVER.

I switched all pre and power tubes and it still does it.

I have pulled the chasis and everything looks in order.

I tried the FX loop trick with no luck. I've sprayed all the jacks with contact cleaner.

I have a voltmeter. Where do I start?

 

[boogiemon]

i'm guessing the filter capacitors need replacing.