Due to the way the GLM boards are designed, there are two receivers tied to a input channel on the micro. Normally this isn't a problem as the transmitters are turned on one at a time, so only one of the receivers is active at any one time.

However, if you have marginal receivers (I'll explain more on this later), one may be adding just enough current to cause a mis-trigger in the input circuit. The effect is that when one opto pair is blocked, the other receiver on that channel can, under the right circumstances, inject enough current to cause the input circuit to "unblock". This is how the micro can determine whether or not there is a stray light source affecting a particular input circuit (but also why you almost always have the red LEDs blinking in pairs - those two are tied to the same circuit).

Usually the way you can determine whether or not you have a "stray light" issue is to unplug the transmitter opto harnesses from the board (in this case, JP2 and JP3). Ignore all the rapidly flashing red LEDs and look at the green LEDs. If any are even slightly illuminated, then the receiver is either getting stray light from some nearby lamp, or the receiver itself is not turning off completely.

At this point, without having the game in front of me, I would suggest taking a look at the receiver optos on the affected circuits and replace them with good modern receiver optos. Stay far, FAR away from "factory original" optos as we have found many of these to have too much light sensitivity (meaning you will likely make the problem worse!).

Hope that helps!