Bare in mind this may not pertain to you as I don't run mame at all...only arcade pcbs...
that being said, I have a monitor that, upon initial startup does not have proper v.hold(that is to say it rolls from top to bottom)...after it warms up, however, the v.hold locks in and everything is fine from then on. On the other side of that, if you re-adjust the v.hold when you first turn it on to make it stop rolling, then after it warms up it will start rolling because you readjusted it to begin with....
so I guess what I'm getting at is have you let it sit a while without readjusting to see if it corrects itself after a few minutes?
Like I said may not pertain to you but just in case thought I'd throw it in for you to try.