System 1's are notorious for bad grounding and even worse connectors. I have a Sinbad awaiting me now to do the ground mods and change out the connectors. I would lean toward's the connectors but definitely you need to do the ground mod for sure. Try reseating the connectors just to see if that helps the symptons but chances are, there going to have to be changed out. They can cause all kinds of odd behavior
http://www.flippers.com/gottlieb_ground_cures.html.
http://www.pinrepair.com/sys1/index.htm