I've go a strange problem with my 99 328i. Symptoms are as follows:

• slightly rough idle, not as smooth as normal
• no error codes
• after running at speed (70-80 mph for 10-15 minutes) and then either coming to a stop or having to slow for traffic makes the car run very poorly for the next couple of minutes with pronounced stuttering or misfiring. this will clear after a minute or so and the car runs normally again. you can run at speed for 10 minutes or 30 minutes - the rough running will occur either way. while at speed everything's fine and the car seems to run strong and smooth.
• in 4th or 5th the car won't pull cleanly below 2000 rpm but is fine after that

I'm a bit puzzled. The lack of error codes for such an obvious misfire is odd - I would have thought the ECU would sense something was wrong.

I will check the fuel pressure as best I can - can't really do that when the car is moving but at least at idle and with and without engine vacuum applied to the regulator.

Thanks Charlie. I have the Bavarian Technic SW and lead so will have a go with that to see if it reveals anything.