Minor V-Hold issue

[FrizzleFried]

I have a PowerMax 1000C (Wei-Ya) G07 replacement chassis...re-capped.   When I first fire up the cabinet,  the monitor,  at 640x480 interlaced (font end) is just a HAIR out of V-hold so it vibrates and makes everything difficult to read.  If I adjust it just a hair, it's fine.   The problem then is that after it warms up (a few minutes),  when I exit a game back to the front end,  (640x480) it is back to vibrating again and an adjustment slightly the other way corrects the problem where it will be fine as long as the monitor is warm.

If I turn on the cabinet and DO NOT adjust the V-Hold,  after about 7-8 minutes when I return to the front end (640x480),  all is fine.

Is there a way to correct this?  It is not a HUGE issue as I have a remote board for the chassis that has a vhold,  which is very accessable if necessary...but I just usually jump right in to a game for a few minutes without adjusting...and when I come back it's fine.

The problem only appears at 640x480.  I can also adjust to a "SEMI-HAPPY" medium where it is just slightly out of adjustment both cold and hot,  but that's not desirable.

Thanks!