I have cleaned the power connectors on both boards (the pins), they were really dirty. However the problem remains, and the machine is reseting sometimes. I activate the pop bumper by itself, and it will work fine, the sling shot will not fire. But if I knock some targets down and activate the pop bumper, the slingshot will fire instead of the pop bumper. Up until now I have been talking about the right slingshot. But I can also get the left slingshot to fire whilst activating the pop-bumper, by flicking the spinner at the same time. The little power board above the transformer (its a bally board actually), has rust near some solder areas, and I reckon it needs an overhaul.

I didnt clean the smaller connector pins on the MPU board, it that worth a shot? ( I did reseat them though). Now that its reseting I will not play it 
 

It's the smaller pins that may be causing the problems... it is a quick and easy thing to do, before you get stuck in the gory details of switch matrixes and other hell...

Double check the "rust" on the rectifier board. If it is like many I have seen, it may be brown crap from flux rather than rust - unsightly but not harmful. That said, it is the "most hacked" board on these machines by far and might need an overhaul, but at this stage it is not causing any of the problems you are currently having!

Cheers
Jacob