OK, finally figured out how to make my machine rest and lock up. When I press both flippers at the same time, whamo. What's the fix for this?

Power supply is actually a brand new modern power supply drop in replacement. Unless you mean the big blue filter cap. MPU and Driver board have already been updated and confirmed working properly. I seem to remember having a friend that had a similar issue but with a more modern pin. Had to have the Bridge Rectifier replaced to fix it. Maybe?

Name of game, manufacturer and year: 1980 Firepower
Did it ever work in your Possession?: No
Did You Run the Diagnostics?: Yes, completes all diagnostics successfully.
Where did the game come from?:Unknown, Seller comes across lots of pins and just turns over what he doesn't keep.
Have needed MODS been done?: Yes, MPU and Driver boards have been completely updated and verified by JohnWartJr
Are any aftermarket boards installed?:Yes, aftermarket PS replacement
What was the last service you performed?: So far replaced 2 known bad coils, repaired/replaced boards. Current troubleshooting leads to activation of both flippers causes game to shutdown/reset (Lock up? All power to the GI remains on but the displays go out and the game is unresponsive).
Battery Corrosion: No, batteries upgraded to Li-Ion
LED or Incandescent lighting?: Currently LED
Blown fuses: All fuses checked for continuity while removed from circuit
Smell:None
Obvious Past Work Performed:Flasher bulb warming circuit disconnected from ground
Do you have a Volt Meter?: Several
Can you solder?: Like the wind
Location: Milwaukee

I have ordered a replacement Bridge Rectifier just to see if the issue resides there. Thanks for your help.

If I had a bad flipper diode, wouldn't the issue show itself during the solenoid test? Why would a bad diode on 1 flipper cause the issue to only present itself when both flippers are pressed at the same time?

Yes, I have 5 of 6 screws holding the driver board in place. The space connectors are snug on the bridge rectifier. I didn't actually run a meter across the bridge, but intermittent issues like this that I had with a friends Roadshow had similar symptoms but the BR tested OK, so I opted to just go and order the replacement.

Thanks for the replies everyone, just making sure there isn't anything else I should be checking that I may have overlooked.

I did make sure I only had the 2 recommended screws in the boards, but still having the issue. Ordered replacement solenoids for the flippers, just going to replace them both this evening. Also ordered a Rottendog replacement power supply (currently has a Gulf Pinball drop-in replacement) to verify whether the PS has anything to do with the issue (all test points check out), and that should be in Monday. The Bridge Rectifier for the solenoids has been replaced, but didn't help the issue. Anything else I can check/test in the interrum?

Replaced BOTH flipper coils, still same issue. If both flippers are pressed at the same time, game freezes. Guess I have to wait on the new PS to see if the one I have is insufficient.