Well I'm hopefully getting close to solving this. Here's the car/problem. '88 IROC convertible with 400 small block/TPI, mild cam, headers and a different rear end that I haven't identified yet. I've only had the car a week so please bear with me on knowing all the history. The car sat for a year without being driven. After that year the guy that owned it couldn't get it to idle. I bought it, changed the IAC and it idled pretty good. I'm changing the plugs (5 down 3 to go).
So.. it runs great when I get it started (hard starts) and seems to be running perfect with a rock solid idle. Then all of a sudden it starts to stumble and SES light comes on. The first time I limped it home and pulled 7 codes, so I figure some were caused later after the initial problem. So I re-set the codes, drove it later and it ran great, full power range, great idle, and then SES light and falls on it's face. I stopped pulled the codes and got a 33 and 51.
My question is will anything else cause the code 51 besides the Memcal or prom? I know the 33 is maf, I'm just trying to see if the computer is possibly bad first. To add, I reset the codes again, drove it for 20 min or so and ran awesome. I wake up this morning and drive not even 3 miles and Mr. SES shows his face and this time I pull the codes and again 7 or 8 codes.

Idea's ???? Hope someone can help.

 

I also have a Jet chip that I got for free with the car. It was not installed though. I put it in to see if it helped but it worked for about 10 minutes of driving and SES back on with 7 more codes.

Something I'm not sure about is the chip goes onto the stock one. Does it still use the stock Memcal? So could the prom be good and the MEMcal be bad is my question.

IS there any way to test the ECM itself while it is out of the car?

BTW, code 51 is always present with the others. If that helps.

Thanks