My 92 GMC 1500 Automatic runs strong from a stop in 'Open Loop'. In "Closed Loop' it runs very lean, low power and stuttering. If I keep my foot in it, at about 2500 RPM it pulls strong again.

I have replaced the O2 sensor, MAP sensor, TPS, EGR valve, plugs, wires, cap & rotor. I measured the Temp sensor & its in range cold & hot. I also added a performance chip to the ECM. If I disconnect the temp sensor once warm, it goes rich and will die at idle as expected. At idle the O2 sensor voltage swings from 0.1 V to 0.8 V. I tried adding a 780 Ohm resistor to the Temp sensor ckt, it will raise the idle and lower the strong pull to about 2000 RPM but it bogs even worse when initially flooring it. If it were fuel pressure, it wouldn't pull strong above 2500. BTW the old plugs were clean, a light tan colored insulator.