1990 Oldsmobile Ciera erratic run

This car was purchased w/40k miles, had excellent maintenance and had a new computer installed, as well as an intake manifold gasket - service performed by GM dealership. I have kept the oil changed regularly and other tune up as well.
About a month ago I started noticing a slight change in RPM while in town - about 25-30mph in high gear and when I stopped, some slight surging. I replaced the O2 sensor 6 mos before because it broke (blew out of the hole).
The codes are 12 only. Not showing any other trouble codes.

I thought about the iac and pulled it, cleaned it. Didn't look bad or very dirty. Also sprayed the throttle body with injector spray. No change. Have checked plugs - NGKs look great after 10k, new plug wires, replaced fuel pressure regulator and yesterday (unrelated maybe?) A KNOCKING serpentine tensioner roller/bearing. I now notice that when the engine is started and cold, it runs perfectly smooth. Within 2 - 3 minutes, as the engine warms and I hold the accelerator @ about 1/4 throttle, it begins missing - just a bit at first, then progressively worse until within 5 minutes it is very noticeable. On acceleration on the road it feels good, tho some loss of power over previous good running condition is noticeable.
I have not taken any voltage readings (I have a digital VOM) or values, tho I'm suspecting something that is affected by the temp sensor - which I think would be the throttle pos sensor. But can't afford to just start throwing $ away on things without some verification. The replacement of the fuel press reg was because I'd had a Ford truck that acted very similarly. Your help is appreciated and I have no objection to making a donation. After reading a few posts here, you guys deserve the support! Thanks!