Neo Geo MVS sync (?) issue

I have a 4 slot MVS that appears to be working fine (diagnostic BIOS doesn't detect any errors), apart from what appears to be some sort of sync issue (check the video below). The sync line has 3V and the two 100 and 480 Ohm resistors on the sync line seem to be just fine once I measured them. The supergun I'm using works absolutely fine with an MV1C and MV1A board I've tested. Any ideas what I could try to fix it before I stick it on eBay for spares?