i picked up an old 2 channel Dual Rec (Rev F) awhile back.
the last three rehearsals i did with it, i had the same problem occuring at times. out of nowhere, the volume would fade/ drop almost like i was using a volume pedal... put amp on standby for a bit, turned it back on, everything fine. i've never experienced this issue with any amp i've ever owned.
could this be something as simple as a tube issue? i really hope it isnt something serious, as i really LOVE the way this thing sounds.