nothing in your post indicates you have any sort of conversion happening to convert the 31khz output of the pc down to 15khz that the arcade monitor runs at--IF YOU FEED IT A 31KHZ FOR ANY ENGTH OF TIME YOU MAY KILL IT. you can't just hack the cable and hope that it works.
if you use an arcadevga you can do away with the jpac as it does all the conversion for you, but with a regular pc video card--forget about it, unless you have something to drive the proper signal.