6 slot motherboard vertical synch issue

So I recently try-synched a New Astro City cab, the previous chassis was totally shot. Anyway the short and long of it.....The cab seems to have a problem holding a vertical synch with the 6 slot motherboard, it works fine with every other game I have, so Im assuming its an issue with the 6 slot motherboard...Any thoughts on how to remedy this problem.

Sync signal may be too strong. You can try a 250-500 ohm pot on the sync line. This is of course assuming the board is working 100%.

Providing a screenshot would be very helpful, however the first thing I'd do is to try inserting a 0.1uF cap in the sync line.