http://antelopearcade.com/forum/yaf_postst4_Pinball-2000-AT-to-ATX-Power-Supply-Replacement.aspx
Also ...... The PC inside a Pinball 2000 game always runs at 120v AC as it gets it's power from the transformer in the pinball cabinet. So if you are in a country that uses 240v DO NOT plug in the PSU directly to the mains socket with a PC mains lead. If you need to run the computer out of the game to do diagnostics on the motherboard, check that the switch on the back of the PSU is at 240v before plugging in into the mains! Also remember to set any PSU back to 120v before installing it in the game and powering up the pinball machine.

I thought the original PS was hooked up in that way.

I would still try plugging the new power supply into the mains to be sure that the transformer in the machine is not causing the problem. if it still does the same thing with it plugged into the mains then you can cross that off the possible problem list.
We don't now what the max wattage load is for the 110v winding on the transformer is so the new power supply being of a higher wattage could be overloading it and the voltage could be dropping enough to interfere with the power supply. It will depend on how efficient the new power supply is.

Is the new power supply designed for 110 and 240v or is it dedicated 240v?