1981 E21 318i idle problem

[erichsbm]

After a short 20 minutes drive one cold morning I' ve noticed after reaching my destination my E21 BMW 318i engine's idle speed has drop down to 400 RPM and has stayed like this ever since. I' ve also noticed when the car is stationary and the engine's rev is slowly increased to about 1000 RPM it seems to jerk a little but if I increase the rev further it runs Ok.
I have replaced the Warm Up Regulator but no improvement. I have also check all vacuum hoses, spark plugs and leads but cannot find fault. Can anyone please give me any suggestions or ideas of where to start looking and how to rectify this or maybe give me a list of possibilities that could be causing this problem?
I will appreciate it very much. Thanks.