The problem could be some other electonic device generating noise/transients on your mains network, as in here. Below are some suggestions which could help out finding the problem (while keeping your consoles directly connected to OSSC):
-try hooking all your AV equipment to same power outlet if they aren’t already
-try disconnecting OSSC 3.5mm audio out if you use it
-after OSSC has synced, “lock” the mode by going to “fw update” screen (Update? 1=Y, 2=N”) with an image on the SD card. Don’t initiate the update, though, being in that screen just prevents mode change if e.g. I2C data gets corrupted due to noise.
There will be yet another sync filter setting on the next fw, but your case sounds like noise gets generated externally and not by the systems itself.