I looked back through your posts and didn't see this specifically...

If you exit the test and then try it again immediately it does it continue to run slowly? If yes, what about if you give it a few minutes to cool down?

So is the motor exhibiting the slow/sluggishness when this happens?

...

I just went through my Fish Tales this past month because the Reel was all wonky. It looked like a switch problem but turned out to be a combination of a failing +12 dc supply on the Power Driver board and a switch matrix issue on the reel opto column (optos also use +12). I ended up having to rebuild the power section on the Driver board because of poor quality of previous repairs (destroyed traces/vias and shotty solder work) and fix bad connections on the IDC connectors to the CPU board.

So.. Check your +12 power supply (this drives the reel motor):
Hook up a volt meter to the +12 input to the EMI board, (use your original board since the new didn't change anything).
Note the reading.
Is the +12v reading good or low?
Run the reel test until it starts misbehaving and see if the +12v dc rail starts to drop.
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So if I understand you correctly here... you're saying that when the reel is exhibiting the sluggish or incorrect movement the +12v power on the EMI board drops to roughly 2v dc?

If you took the +12 measurement on the motor board move your DMM up to the Power Driver board and perform the same test to see if the PS is dropping or if there is an issue in the wiring to the EMI board.

Resets are common on WPC games when they have some age on them. The correct course of action is to go through all the steps (in order) outlined in the PinWiki.

https://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets

I've had bad MOVs, connector corrosion/burning, and all sorts of messed up repairs in the power section in my collection of games.

Glad that seems to have corrected the reel problem.