Curious: did the joker valve prove to be the problem? I'm having a backflow problem myself -- when the boat is at rest! Leave the boat for a few days (this week, for a few hours) and the head is full of water (or worse, overflowed) upon return. First time I've had any sort of issue since buying the boat, so I know the vent loop is configured properly and that the lines aren't clogged.
1980 Tartan 37 Hull #228
Harwich Port, MA