...and to add one more theory to the pile.
My Space Wars had the same symptom. Something around the 7406 on the
monitor board was buggered up. I never did find it, but when I lifted
the pin on one of the 7406 inputs the problem went away. (I think their
equivalent of a "spot killer" was turned on for some reason. Seems like
it monitored the voltage supplies.)
Something else for you to look at...
-Clay