Randall RG100G3 dropping volume

[Christopher Wargo]

Hi gang. I recently bought a used Randall RG100 G3 combo amp. I've been playing through it for a few weeks. A few nights ago the volume started dropping out on it. It seems to happen randomly. It drops to barely audible for a minute or so. I might jiggle the cable or the knobs, but that doesn't help. Then it randomly comes back.  I plugged directly into the amp just to make sure it wasn't any of my pedals; same problem. I also plugged into another amp to make sure it wasn't the guitar, and everything was fine. I haven't turned up past, like, 3 or 4, if that matters. I have gotten conflicting info online, so I'm not sure if there's a tube in there or not (I can't see one when looking in the vents, at least).

Any ideas here? I'm not too savvy on this stuff, so I'll likely take it in to someone who works on amp, unless yall think it's something relatively simple I could do on my own.

Thanks.

edit: Oh, I tried plugging into all the channels (2 gain and 1 clean) and the problem persists.

[g1]

  A simple thing to try is using a spare cord to connect the FX loop send to the return.  Does that make any difference?