Slight prob with amp/pre

Hi all

One of the set-ups I'm using is *gulp* solid state , actually I really rather like it!!

Slight prob; I like to leave the pre switched on but turn the power amp off when no in use. If I leave it like this for a few hours when I switch the amp back on a tend to get what can only described as oscillation. If I turn pre off I dont seem the have the problem!

Ok, simple answer is turn the pre off but what could be causing the problem in the first place?? I've gone over everything many times, I've used 1R resistors in certain areas of the grounding and ensured that all components are with tolerance. I've now run out of ideas!