Help!! Mark III with Low Volume When Using FX Loop

Hi guys! I've a problem with my Mark III. I was playing a bit this afternoon and everything was normal. I switched off the amp, and a couple of hours later I turned it on again (without changing any setting or connections of course) and the volume was really low, so I panicked a bit. I removed my G System from the FX Loop and plugged straight into the amp, and the volume was back to normal again. However, I'm sure it's not the G System or the cables because I plugged in my old Nova System with a different set of cables and the problem was back (really low volume, and a bit of extra distortion).
What could this be? A preamp tube maybe?
I'd really appreciate your help!

Mine was doing the same thing. I read on here that it could be dirty send/return connections (among other things) so I used some contact cleaner on them. They seem to be working OK now using my Boss delay and chorus pedals.

Thanks for your help! I spent a bunch of time yesterday night and this morning searching the Board looking for possible causes. I haven't found posts about Mark IIIs, but I've found some about Mark IVs and some about Rectos. I'm going to see if the problem persists if I plug a patch cable from the send to the return, and spray some contact cleaner onto a plug and work it a few times in and out of the jacks. If that doesn't solve it, what should I check next? Could it be a preamp tube (maybe V2)? The amp is 22 years old and still had the original preamp and power tubes when I bought it two years ago. I changed the power tubes, but the preamp tubes are the stock ones.
Thanks guys! Keep 'em coming!

Edit: I finally solved my issue today. Tried plugging a patch cable from the send to the return and the problem wasn't there. Ended up being a mis-configuration of my G System, fortunately. Thanks!