Has the mpu been serviced at any point? I don't know much about Game Plan games, but i read that there's a capacitor near the red push-button switch on the mpu, below the right-most dip switch bank...It's a 0.01 uf/16v ceramic cap that is involved in the cpu timing. Apparently can cause weird behaviour if it goes bad. Not sure what else to suggest, hopefully someone more knowledgeable can chime in.