Here's some info from Clay which may help:
Wacky Switch Matrix due to Low 12 volts.
On some WPC games, in particular Theatre of Magic, a weak BR1 or weak 7812 voltage regulator on the power driver board can cause some wacky switch matrix problems. For example, if the game is doing a "slam tilt" or randomly firing the slingshots or flippers or other coils, the power to the switch matrix may be weak. The 12 volt fluctuation for the switch matrix can often be seen when many of the feature lamps are on. Try removing connector J133 (disabling the playfield controlled lamps) and see if the problem goes away. If it does, rebuild the 12/18 volt power secition (BR1, C6, C7, the Q2 7812 voltage regulator), and install jumper wires from BR1 to its associated filter caps (as described in the reset section). This bridge rectifier and voltage regulator supply the power for the switch matrix.
For example, wacky game behavior on Theatre of Magic (ToM) can be caused by low or intermittent 12 volt power (usually due to a bad or cracked traces around bridge BR1). Maybe most all coils fire like slingshots, magnets, pop bumpers, and the low (hold) voltage on flippers, or even a slam tilt.
Phantom Switch Closures: a Shorted or Mis-Wired Switch.
It's a strange problem. While playing a game, the ball goes down the right inlane, and the left slingshot fires! Or when making a ramp shot, the game slam tilts. One switch closes, but a completely unrelated event than occurs.
This is a classic problem of a shorted or mis-wired switch. It confuses the switch matrix into thinking something else has occurred. This can happen from an "air" pinball, that bashes an above playfield switch's contacts together, causing a short. This is very commonly seen on say Indiana Jones, un the front right side of the Path of Adventure, where the switche contacts are exposed and easily bent together by an air pinball. This problem can also occur from an improper repair where the row/column wire is wrongly attached to the switch and/or switch diode. Also a bad switch diode can do this too. In any case, the problem switch needs to be found. Unfortunately, it won't be obvious. The switch matrix is confused, so any diagnostics the game provides will be of limited help.
First, try and find the "phantom" switch (the switch that causes something unrelated to happen). Take the playfield glass off, and start a game. Activate the switches with your hand, and find the phantom switch. Once the switch is found, go to the game manual and find the switch's number, row number, and column number. Say for example, switch 53 (column 5, row 3) is causing the phantom closure. Now get the other three switches that make up the "square" of this row and column. First get the reverse switch number, switch 35 (column 3, row 5). Then get the other two switches: switch 33 (column 3, row 3), and switch 55 (column 5, row 5). The switch short will probably be one of these four switches.
For example, if a row wire is attached to the wrong end of the switch's diode, the following can happen: If the mistake-wired switch (#1) is triggered and another switch (#2) on the same column is triggered at the same time, then another switch
(#3) on the same row as the mistake-wired switch #1 is triggered, the switch (#4) on the same column as switch #3 and on the same row as switch #2 will also shows as triggered, even though switch #4 wasn't actually triggered. (Wow, that was confusing!) For example, row 3 column 3 (r3c3) is mistake-wired as described above. If r3c3 and r2c3 are triggered simultaneously as well as r3c6, then r2c6 will also show as triggered (falsely).