I recently experienced the same thing with my pre-amp. I also have experienced this with my older tube amp. It was a loose tube socket.
Have you cleaned the FX send/return inputs?
How good are you with electronics? If you are not familiar with electronics troubleshooting, you can really hurt yourself.
If you decide to troubleshoot, be very careful.
MIA P & J, Jule Monique Rack, Crest ProLITE 2.0, Thunderfunk TFB750-A & 550B, Aguilar GS112 stack, fDeck Series 3, Thunderfunk Club #35