Need help with diagnostic PLEASE!
 

I've got an '89 caprice 9c1 (5.7 TBI) and it has been running poorly. I gave it a full tune up, but it still feels like it's "missing" regularly through all rpms. When I changed the plugs, two of them were all black and sooty (I even checked the firing order - no dice). No vacuum leaks, fuel seems to be flowing...

Finally I checked the trouble codes and it's flashing "42" , which my Hayne's manual treanslates as a problem with the EST, or electronic Spark timer. They also say this could mean I have a faulty HEI. What they don't say is what to do about this. How do I proceed? How can I check the HEI?

Any help would be greatly appreciated, thanks