If I am understanding you correctly, and your amp is dropping in volume,fading off and coming back, but not totally going silent, I believe you could have a power tube going bad, as time goes on it will probaly happen more often with greater effect, the tube is dying off slowly and might just drop dead at any moment, best bet is to get it to a shop as soon as possible, keep the best matched pair of tubes for a back up set, get a good quad and get the bias set and you probaly won't believe how good your amp sounds...........

btw, I'm not saying it couldn't be something else, but this is probaly the most likely and about as good a diagnosis as you'll get on the net.....