You say the vol is dropping out and then returning. When the amp is opperating normally i.e. when the sound comes back up to normal level, try giving the head a sharp whack with your fist to see if you can induce the vol drop. If you can there is a loose connection somewhere. Try plugging a patch lead from the effects send to effects return. If this cures the prob then the contact braekers on the jacks need cleaning. Plug your guitar into the effects return, Does it happen now? if not the prob is in the preamp. You need to isolate where the prob occurs and the above checks should do that.