ArcadeVGA / JPAC loses sync (defective JPAC)

[XYXZYZ]

I have an ArcadeVGA connected through a JPAC to an Electrohome G07, MaLa as a frontend for MAME 123. Everything was working just fine, but one morning I power everything up, -haven't changed anything- and now whenever MAME switches the resolution to play a game, the horizontal sync goes out. (The "sync OK" LED on the JPAC goes off) The screen displays a double image kind of thing with a warped second image just to the lower left of the first image. The sync is still out when I exit to windows, but if I put the PC to sleep and wake it up, the sync returns. It even works in MAME, if I set the PC to sleep while a MAME game is running, the sync is restored when I wake it up... for about 20 seconds, then the sync goes out again.

Every now and then it won't do this, MAME games work normally but usually lose sync during the game at some point. And sometimes the sync OK LED on the JPAC just flickers on and off, the screen jumping around accordingly.  The INI settings for each game don't seem to have any effect on this, settings that once worked, and the MAME auto settings give the same results. And I've tried using different MAME versions, no luck.

I have no idea what to look for, do you?

[XYXZYZ]

Okay, I switched the JPAC with an older JPAC I was using an another machine. Everything works flawlessly! I put the suspect JPAC in the other macine (Matrox G400 card) And I boot to the frontend, turn the monitor on and it works fine.... for about 5 seconds, then all the sudden the sync OK LED goes out and the picture starts jumping. So, apparently I have a defective JPAC here.

I'd kinda like to attempt some troubleshooting before I look into returning it, but  again I'm not sure what to look for.

[BASS!]

Talk to Andy, he is possibly the most down to earth and helpful person you will meet.