After going through all typical pressing on socketed chips and re-inserting ribbon cables and other connectors, if problem persist you need to isolate to what exactly causes the reset. If you manually press switch #56 , Left Juggler, when the game is in attract mode, does it reset ? I don't know this game , but does a coil fire when you press that switch ? If it didn't reset during attract mode (because it won't trigger anything else) then it's most likely not the switch. Then I would try in switch test mode and see what happens then. If it's the switch directly , unplug the connectors at J-206 (or J-207) and J-208 (or J-209) then use a diode and jump J-206 pin 5 (banded side of the diode) to J-208 pin 7 and see if it resets then. If it still resets the problem isn't in the switch or it's wiring. If it does fire a coil when that switch is pressed , then disconnect that coil and try the test again.

The idea here is to check if closing the switch resets the MPU or a coil firing is resetting the MPU. If you disconnect the coil and the game resets with the diode, then some component on the mpu board is broken. It is more likely an issue with the MPU board than the PDB , and you've already swapped the PDB to no avail. I wouldn't be surprised the problem is u19 since it is connected to the 5v line.

All this is of course assuming your problem is not damaged power connectors at the PDB and between the PDB and MPU board AND that it isn't corrosion related on the MPU board.

Can you post close up pictures of the MPU board from battery holder all the way down , including the connectors?
Disconnect the switch matrix once the game is in play mode and close the juggler switch with a diode, does it reset ?