Do the pins reset when only both are on? Have you tried different outlets/circuits?

I would suspect low voltage from the outlet. Put your DMM in to AC mode and see if you are getting 120 VAC from the outlet. If you are too low, your transformer will not provide the correct 5v to your MPU and you will get resets as the voltage fluctuates during game play.

Sounds like you have very inconsistent voltage in the building. An overloaded transformer perhaps? The landlord may not even have any control over it.

Use a multimeter and check the voltage of the socket to see if it's too low.

I'd also say try measuring the amperage, but I think most multimeters max out at 10 amps.

Also: http://pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets

It could be bad input voltage. Go get a UPS from the office store and plug that into the wall and the pins into that. It should at least stabilize the power. You may need one ups per game since there could be too much draw otherwise for the battery to keep up.

The stern probably pulls 3 amps and the Tom probably 5 amps. 3 amps is 360 watts and 5 amps is 600 watts. Something that can run 500 watts for 5 minutes should be sufficient once you let it charge up (plugged in with pinballs off until it's charged). Since you just want to stabilize power and don't want to actually run the games on battery, you should be able to find something relatively cheap to handle the above loads. The boxes will tell you the loads the ups handles. So take the above wattage info to make a purchase

when i lived at my old place the power was shared off the people that lived below me.

I never had resets but my games seemed to played weak

When we bought our house i had the power updated and could notice the difference, The ramps were easier to make on my BSD and my pops fired like crazy on firepower

Unfortunately, this doesn't mean much since it's not under load. Try a cheap ups and report back.

Yea that should be enough for what you're doing.