Proves that the power supply connectors are fatigued. Very common issue on Bally's and Stern's of this era. You have two choices, repin the connectors and the board itself, or buy a new board. But you will still have to repin the connectors. The power supply board can be repaired / upgraded - have a look here ;

http://aussiepinball.com/index.php?topic=1557.0

New board can be purchased locally

http://www.rtbb.com.au/catalog/product_info.php?cPath=22_86&products_id=1128

If you need to "get by" for now, grab some sandpaper and run it over the connector pins on the power supply.