http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets

Go through the reset list. Could be a few things to consider.

LTG : )

Check for +5 volts on your MPU. anything less than 4.85 is apt to cause a reset. The reason for a drop in voltage is most likely a weak connection such as stressed female connectors and cracked solder joints on the male header pins. Another possibility is chip sockets on the board itself. Over time, the chip's legs can tarnish and/or the socket can lose it's grip on the chip, allowing a poor connection. And battery corrosion can be a problem as well. I don't suspect that here so much because you said messing with the connectors temporarily remedied the problem.

Most often you will get these resets when you hit both flippers at the same time. As Loyd said, there are lots of different things that can cause this. Try taking off and reseating the high power connectors in the top right of the power driver board. If this provides temporary relief, you have connector/header that will need replaced, like smokey suggested.
-Brian

Your game is resetting cause it wants the new LCD hologram mod!

Going below 5v or Bridge Rectifier. I had an issue with my TZ doing the same thing. Switched the one in my BSD with it and never had an issue. Some pins just require more juice i guess. TZ was getting 4.87 and BSD was getting 5.1