I have described problems with my Ipac in another post. I get weird results (random movements) when playing games.
I had a question about a ground loop - when I wired up my CP, I had one wire from the Ipac for each player's controls. The wire went to the coin button for that player. From there, I made a loop to the player's controls. This is not the same as a regular ground loop since the loop actually starts and ends at the coin switch and not at the ground terminal of the Ipac. Could this be causing the problem? I'm not real familiar with electronics, but I thought this would work.