1991 GMC C2500 OBD1 code 85 - prom error

I have no clue what the code 85 - prom error means, the truck has an intermittent misfire, it rev's up at times and stalls at others, I have replaced the ignition control module, in the distributor, and it seems to have fixed about half of my problems, but the truck still is not right, is there anything you can do to help shed some light on what else the, 85-prom error, means needs to be replaced? I have entered into a realm beyond my knowledge, so any help is greatly appreciated. Thanks Ray