My 1991 940GL with B230F and Bosch LH2.4 has been exhibiting a bad cold idle problem. The car starts easily but will immediately run rough in chug-chug-chug-chug fashion. The tachometer stays fairly stable during this time. Occasionally the engine will rev up VERY briefly by about 200rpm as though something is trying to compensate for the problem.
The car had the problem a couple of years back but seemed to fix itself. Firstly it started doing it again intermittently but in the past week has done it everyday both in the morning after sitting overnight in the garage and in the afternoon after sitting all day at work (about 8 hours).
The weather has cooled down here fairly quickly in the morning but I don't think ambient temperature is a factor.
I'm thinking maybe it's a faulty injector, valve deposits or faulty coolant temp sensor. I'm going to test the sensor this morning. There are no vacuum leaks as far as I can tell and items such as ignition leads, distributor cap and rotor and spark plugs were replaced last time I had this problem - nothing fixed it. Is there anything else I should look at?
Thanks for any help.

Brad