Reminds me of behavior I'd get on my BSD when there was a bad diode on a switch. My game would act possessed and fire upkickers seemingly randomly. A friend of mine, who was much better diagnosing issues, chased it down to a bad diode on a switch. When pressed, it would cause chaos on adjacent switches in the switch matrix. Maybe take the glass off and press someone switches while the game is running, see if that causes your possessed drops?