I've got a BBE sonic maximizer that i like to run in the effects loop on my setup. It does this weird thing after a few minutes where the sound starts getting fizzy, cutting out and getting volume drops.
My only thoughts are is that you have a bad capacitor(s) in there that are going bad and after they get warm from being on, they start crapping out. I work in IT and I ran into this issue when working on some older dells. After it was on for a while, they started blue screening like mad until they were unusable. I googled it and found it was common with that model because of a bad shipment of capacitors on the motherboard. Maybe something like that is going on here?