Which kind of circuit you use to split sync to H+V? I’ve personally used ISL59885 in similar applications where it has worked reliably, and it was ok with OSSC too when I tested it. Are these systems otherwise directly connected to ossc or are there other devices in the chain? There has been a couple cases where a bad quality PSU (not necessarily the one powering ossc) in closeby outlet has caused bad enough interference to screw up sync. I’d guess your problem is either that or a defective board.