Vertical Hold Problem, only with 1 Jamma board, not 2 others

[Casper]

I'm hoping you guys will have a couple ideas on this one. 

I have a Mortal Kombat II cabinet with a Hantarex monitor (the picture should be attached).  The cabinet/controls/monitor all work great with the Mortal Kombat Jamma board or the Tekken Tag Tounament Jamma board.  But when I put in my Street Fighter 2 jamma board, it plays fine for about 45 minutes, then the vertical hold starts to give out and I can't get it back. 

I can shut it off, put in the other jamma boards and the picture is fine again. 

Before you say it's an issue with the SF2 jamma board... I also tried a second SF2 jamma board and had the same results (one was SF2 the original version, one was SF2 Champion Edition). 

My service guy checked all the loose wires, did some soldering and fixed or checked everything he thought would make a difference. 

Are the wiring or power requirements different for these boards?  Are they not compatible somehow?  They're all jamma and they all work flawlessly.  Flawless until the SF2 loses its vertical hold. 

Any help is greatly appreciated.  Thank you!
(as you can imagine, searching for "vertical hold" yields a number of posts, but none I found are related)

[grantspain]

when you lose the vert sync does the vert hold lock it back into position?
if not try a mod we used to do many years ago,connect a 5k resistor from vert sync wire to video ground on the video input plug of the monitor chassis