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BMW randomly stalling at idle when warmed up

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My wifes 1990 BMW 325is (which we affectionately call the sh!tteroo) stalls at idle. It has a new head gasket, idle control valve, CPS (crank position sensor), coolant temp sensor, plugs, cap and rotor. It did this before all these parts were installed.

When it is below normal operating temperature it idles fine. Once it is completely warmed up it sometimes will stall when idling or if you depress the clutch pedal like when coming to a light. It doesn’t do it all the time. For the last 3 days it didn’t stall at all. Today it stalled 5 times in 30 minutes. Sometimes the RPMs drop to 500-600 for a sec then jump to 1200 before settling back to normal. Sometimes it just stalls out. Sometimes it runs fine. Last week I drove it to warm it up. I pulled into a parking lot and sat for 20 minutes. It idled perfectly for all of those 20 minutes. I drove it some more, sat on the side of the road for 10 minutes and it “dipped” for a sec and was fine the rest of the time.

I have checked all the sensors per Bosch specs (including the TPS). I am assuming that because the car runs great and pulls strong when it is under load, cruising and accelerating that it is not ignition, coil, or fuel related nor is it mechanical (like low compression in one cylinder, etc). The check engine light will come on for a sec but there is no fault stored (maybe just because the low RPM?). I have checked every hose and tube for leaks by putting a tube in my ear and listening for air leaks. I unplugged the purge valve but it still stalled. However, since it only does it once warmed up and it seems like a randomly introduced vacuum leak i’m not sure if that can be completely ruled out yet.

I’m pretty good with Bosch motronic systems but this one has me stumped. Does anyone have any thoughts?