The car starts up fine and idles well for a few minutes, with the O2 sensor reading about .45v, but after a few minutes the rpm will start to fluctuate, and the O2 reading will start to drop to about .25-35v and then come back up to .5-6v. The main problem is that sometimes the readings will drop all the way down to .15-20v and this will cause the car to stall before it can recover.

Once warmed up and running the car runs well, I have done a full tuneup recently (injector seals, vacuum lines, fuel filter, air filter, cap\rotor, plugs\wires, intake manifold gasket) and the vacuum is stable throughout.

I did notice however that I need to replace the thermostat, as the car takes forever to warm up now that the weather has cooled down, and when I started it today I could feel warm coolant flowing through the top radiator hose very quickly after startup in 40F weather, and well before the temp guage even began to rise.

The question is, could this be related to a very slow warm up due to the failed thermostat, or could it be caused by something else?

Thanks for any ideas,
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Zack Silver 83' 242Ti (220k)