if it's just resting it could be anything from a I.C. that needs to be reseated to a problem with the power supply or a bad connection.
you will need to unplug and visually inspect each of the larger plugs (I.E. not the ribbon cables) for heat damage. you will also need to do some tests with the multimeter to find out if it is a low 5 volt issue.

If it is a slam tilt then thats a whole different scenario, try triggering the slam tilt with the slam tilt switch and see if it just resets or if it gives you some indication that the slam tilt has been triggered.
   

OK I reseated all connectors including power supply high voltage ones and managed to blow a 5 amp fuse which i replaced and cleaned boards and pushed chips in the only ribbon cable is for scoring the game played OK for 2 games so i'll monitor it but the dodgy switch still doesn't work :( when i trigger slam tilt in door it just stops game doesn't reset and there's only 2 tilt switch's not 3 so all good there. May need to wait for someone to come over who knows how to test for volt issues me I'm not that good 

Peter