You might want to look at the pre-driver chip U1 or U3 on the solenoid driver board if some coils still lock on after replacing the transistors. U2 can also fail causing the coils to lock on... same for the drop target reset coil. I would look at that before fooling with the MPU.

Take a look at pinwiki. There is a section with a nice write up on how to test U1 which is a transistor array. It is a 3081 chip. All you need is a multimeter. Check U3 and U4 while your at it. If those check out ok, then look at U2. U2 is best checked with a logic probe while the power is on.

Don’t worry about the caps unless they’re cooked. Then just replace. they are inexpensive

from Clay's repair site "Now we need to test the chip that drives the diode and driver transistor. This chip is a CA3081 (NTE916) transistor array. Basically it's several transistors packaged in a chip format. This is known as the "pre-driver transistor pack". You can test this too:
Turn the game off.
Remove the Solenoid driver board from the game.
Put your DMM on diode setting.
On the component side of the board, put the black lead of your DMM on the GND test point just to the left of the U1 chip.
Put the red lead of your DMM on the two pins of the pre-driver chip (one at a time) that you noted above.
For the pin that connects to the 1N4004 CR diode, you should get a reading of .1 to .2 volts.
For the other pin, you should get a reading of .7 to .8 volts.
Reverse the DMM leads (red lead now on GND). You will get the same .1 to .2 volt reading for the pin that connects to the CR diode. The other pin will read 1.1 to 1.3 volts. Note this test is far less conclusive than the first test with the black lead on GND.
If you get any other meter readings, replace the pre-driver CA3081 (NTE916)