Quoted from LTG:

flaky diode on a flipper coil

Quoted from wallybgood:

Failed Thermistor

Step 5: Examine the Thermistor

The thermistor is (generally) a black disk, about the size of a dime. If your game has one (and not all WPC games do), it will be connected in series with incoming AC power. It is located inside the power box which is found just inside the coin door and to the right. Note that the power box may also contain a "varistor", or MOV, which is essentially a surge protector. The varistor will be wired in parallel with the AC power. The varistor is not a factor in game resets.

The thermistor's job is to limit current inrush into the capacitors when the game is first powered on. This reduces stress on the bridge rectifiers or diodes in the game's power circuits (which is the primary cause of bridge rectifier failures). After a few seconds, the thermistor heats up and drops to a very low resistance. Failing thermistors pass less current and have to get hotter to work. This heating takes time, so the game will often reset in the first 30 minutes of operation, and then be fine aftwerwards. Obviously, a cold environment will make the symptoms worse, and a warm room may appear to cure the problem.

Resetting while the game warms up is therefore a key indicator of a failing thermistor. Note also that DCS and WPC-95 A/V boards may reset independent of the MPU. If this is the case, you'll hear the characteristic same "bong" as when the game boots.

***Safety Warning*** Unplug the game AND turn the game off before conducting the following test.

Sometimes, the thermistor may be visibly damaged. However, it may look good and still be bad. An easy test of the thermistor is to jumper across the legs of the thermistor with a heavy gauge wire. If the game resets no longer occur, replace the thermistor with the correctly rated part. The original Williams part number is 5016-12978-00. A replacement is available from Great Plains Electronics.