Ultimarc has a "tri sync utility" that you may need. This monitor has DDC, so it will try to tell the PC what to do. Unfortunately, it just usually ends up confusing the drivers in the process.
If that doesn't help, this monitor's sync separator seems picky. Try using separate sync, rather than composite (which is I think what the AVGA defaults to). Either polarity should work, but I've had good results with positive going sync (both H and V).