Preamp hum off/on?

Hi. I have an Eastern Electric Minimax preamp that hums when I power it up, then it may go quiet for a few minutes, then the loud hum starts up again. On/off for hours. It isn't variable-either dead quiet or a loud hum that can be heard 15 feet away. It is mated with a PS Audio HCA-2. I think it is probably the preamp, as the speakers go quiet as soon as I turn off the Minimax. What is likely going on here? I thought of maybe ground loop problems, but the hum would be constant in that situation, not on/off. Possibly a tube issue? Anything else?

I just had the same problem.Unplug everything in your house,even the refrigerator and all alarm clocks.The culprit in my system was my computer monitor and Playstation in standby mode.Those things weren't even turned on!