Watch the Door Close bit in internal memory, during the close cycle. Compare it to the DLK LED on the processor board.

IF the DC bit goes low, before the DLK led lights up, then the processor is ceasing the close function, probably because that's the way it interprets the inputs it's receiving, one of which, (one of many), is the Door Close Limit.

On the other hand, if the DC bit stays high, yet the DKL led stays low, then the processor still wants to close the doors, but it's intentions aren't amking it to the DC coil, and in between there could be an issue.

(Hope that doesn't sound too vague)

Also watch the DCL bit in internal memory. Betcha it's going low!

If my memory serves me right,on that vintage MCE there are two triacs involved in the close ckt. One is DCP door close power and the other is DCF door close function. Make sure both work. The other possibility which I don't think you have on your vintage is a DPM timing problem. Newer jobs use an additional input called DPM (door position monitor) which makes about two inches before the door closes, then you pick up your gate sw and last the DCL limit opens. DPM must be set to make prior to both the above or you get sequence problems. I guess you checked the retracting roller on the GAL clutch for flat spots? The limit flags are easy to watch using the diagnostic dip switches on the swing panel. The only one that can be tricky to find is DPM which you probably are not using on that vintage unit.