1987 gmc with code 42

[jesdude7789]

okay i dont know what happened but basically i replaced my spark plugs, spark wires, and cap and rotor on my truck last week and within the same day of driving it around it started to die on me when coming to a stop at a stop light or at lower speeds in general. then it gave me a code 42 for the EST.
well i checked all my wires and everything was hooked up fine and the timing was set correctly.

I have a spare ignition module so i went to oreillys and tested it first to make sure it was still good and he cycled it 3 or 4 times so it would get hot and it still passed. okay then i swapped it out with the one on my truck and went and tested that one as well and it tested ok also, so both modules were good. the truck still had the same issue, dying on me randomly and setting the code.
i had to drive to beaumont the other day and i had no choice but to drive it and hope it doesnt die. during the drive it died once, then i got it restarted and kept driving it, several miles later it tried to die again so i throttled it and that kept it running but with the check engine light ON the whole rest of the way to beaumont.
while in beaumont i pulled the whole distributor out and replaced it with another one under warranty which is great but on the way back from beaumont it still did the same exact thing.

so... i have a completely new ignition system except for the ignition coil, the wiring that goes between all the components, and the ECM itself.

I'm thinking the wiring pigtails and such are just too old and i should splice all new ones in.

any thoughts?