Salamander playing (almost) blind on NNC candy

I've got my Salamander bootleg hooked up to my New Net City cab via a Franco B jamma loom. 

I can hear all the sounds and at first thought it was playing completely blind so I upped the contrast and brightness as far as they will go on the monitor and I can see the graphics, albeit very faintly. After a minute or two the monitor loses sync (displays no signal) and then regains it after about 30 seconds. 

To test the board I plugged it into my supergun and outputted to an LCD via scart and it's perfect, which would seem to eliminate any board issues. It's also just come back from being serviced by Phil (VectorGlow) and was working perfectly for him.

I've got Street Fighter Zero 3 which plays absolutely fine on the NNC and I've also hooked up a Pi directly to the monitor via VGA, bypassing the jamma loom, and that's fine too; MAME displays and plays no problem.

Any ideas why Salamander won't display properly?

Thanks,
George

Will do but would that not affect my Street Fighter board too? That displays fine. 

That could well be a grounding issue as others have said, and is what I'd check first.

Is it the Tosh D29 in the NNC you have? That's an OSD display - as in the adjustments are 'digital'. Another thing to check (and I can't remember if a D29 does this) is the 'picture information' bit of the OSD - that is to say the info about the signal. Might give some detail on the incoming signal and whether it has actual sync.

Just check if there's continuity between pin 14 (parts side ) with pin 1 (GND on both sides).

Or check if pin 14 has a black wire connected to it-that is usually the colour of a GND wire.