I would first look at the line voltage in the wall to make sure everything there is proper. Then I would look at that same voltage but on the transformer itself ,or at the exit of the DPST switch that brings the voltage to the transformer. If this voltage keeps changing , then I would blame the thermistor and short it out to test.

If the AC voltage of the transformer is good between pins 1 and 7 , I would take a voltage reading of the DC 5v , see if this one is stable (probably isn't). I would look at the integrity of connector J101 on the Power distribution board and make sure it is in good shape, connector , wires and pins on the board.

I would also look at the CPU board underneath the batteries to make sure they didn't leak..

Ideally you want to measure all voltages AC and DC and post them to get a better overall picture of what could be going on, but the thermistor and the 5v line would be the prime suspects.

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Start working through the 'reset problems' section of the WPC pinwiki. You have an immediate reset issue.

OP here. Time for an update! I was logged into an old account when I made the original post.

Voltage readings:
Outlet 117
DPST Switch 117
Pins 1 and 7 117

I'm not sure where to test for DC volts. Going to check through the pinwiki reset area now

Just found a semi burned J101 connector. Ordered a replacement set and backups.

Nice!

You should always go through the whole Pinwiki article about wpc resets, but lately I've had a lot of wpc reset issues fixed when I replace pins and plugs on J101 (where +5v comes in), J114 (where +5v goes out to the CPU), and J210 (on the CPU where power comes in).

I'm a big believer in crimp and stuff connectors with trifurcon terminals instead of IDC type. The current handling capability is the same for both types of connectors, but the vibration resistance is better with trifurcon, and pinballs are almost the definition of a high-vibration environment.

If you only bought parts for J101, it's possible you'll get it fixed just with that connector. Particularly if the pin is discolored due to heat. If one connector fixes it and I didn't have parts in my hand, I'd wait until I had further problems before I worried about the other connectors in this circuit.

I seem to have ordered the wrong connector for J101. Could someone point me in the right direction?

https://www.pinballlife.com/156-396mm-crimp-stuff-7-position-housing-with-locking-ramp-17415.html
https://www.pinballlife.com/156-396mm-locking-header-7-pin.html
https://www.pinballlife.com/156-polarizing-key-for-molex-crimp-stuff-housings.html

The trifurcon terminals:

https://www.pinballlife.com/156-trifurcon-crimp-terminals.html

Get extra terminals in case you make a couple of bad crimps...