B2600i 1992 problem

OK, so the truck was running great and after a few days it has developed and strange issue - before I go hunting it down, I wanted to ask if anyone has had this issue.

The truck runs perfectly well and if you try to gun it or rev it up while cold, it'll come back to idle real low and lumpy, if you continue to drive or hold the RPM up (2500) for a few seconds it'll clear out and back to normal

I opened the ECU and don't see any damage or leaking caps.

Any input would be great.

Clean the mass air flow sensor? Timing might be slightly off at the dizzy?

Maybe your O2 sensor has decided to go on permanent vacation. They work really sluggish when they get old.