Finally I managed to fixed my problems: after getting hot (no difference between open/closed loop, just temperature) my car keep loosing power, idle was hesitating from 800rpm up to 2000rpm, there was some stumble betwen 2500 and 3000 rpm. I also got very bad gas milage and backfireing.
I got some Datamaster readings - there were very strange, while hesitating the rpms would jump to 4-5k for a split of the second. What I did:
- changed the plugs to NGK 55TR
- changed the fuel filter
- put new exhaust headers gaskets (there was small leak there)
- removed the cat (was clogged)
- installed LT4 knock module
- fixed intake oil leak
- removed AIR and EGR
I also checked and cleaned:
- MAF
- IAC
- IAT
- TPS
- MAP
- intake elbow
- throttle body
- injectors

That didn't help. So finally I bought and installed:
- Delco 41-906 plugs
- Delco plug wires
- Delco O2 sensors

Wow! The beast is alive and kicking again! No more hesitating, no more lack of power - it's great, I fall in love with my car again. The main reason was the coil wire - it was arching very badly and O2 sensors were also shot (80k miles on them). I was suspecting wires, but couldn't find any connection between temperature (car was acting good until hot) and plug wires.

I have my old Datamaster readings - if you suspect same problem I can send you mine (or email me yours), there are very characteristic jumping of the RPMs.

 

congrats... its always nice to find and fix a problem.... it seems like thats ALL i do....