1989 GMC S15 Missing throughout driving range.

I've done a complete tune-up, plugs wires, rotor and cap. Changed the fuel and air filters. about a month after this the truck started to hesitate while accelerating and on long trips will sometimes shut off and not start for 10 minutes. I did a computer check and I'm getting a code 42. Probable cause: Electronic Spark Timing bypass circuit or Est circuit is grounded or open, A malfunctioning HEI module can cause this code.

So here is my question. I have the Haynes manual but I can't find out WHERE these circuits are located. Also is the HEI module a fancy way of saying distibuter cap? I need this truck to run and I've spent days on this. I'm broke so I have to fix this myself. PLEASE help. I'm a plumber not a mechanic so please explain it like you're talking to a child.
Thank you.