A few questions that may be relevant?
What transmission do you and all the other owners have, I assume this problem only manifests itself with manual transmission cars?
Also explain what gear you are typically in and what speed the car is typically traveling?
I will relate a situation that I fought with on a very different BMW without a DISA for some time. I spent an awful lot of money and time on the issue to find out there was a big problem with the timing maps for the car. I believe these may have been tweaked timing maps, but not 100% sure if they were bone stock. Note my car has a 6 speed manual transmission.
What I found was between 1500-3000 RPM there was a chuggle, misfire to stutter as you mention. There were NEVER any misfire codes and as soon as the engine spun past 3k RPM problem was GONE.
So if you were driving spirited where you spun the motor up in 1st gear and kept on getting into it, the engine was ALWAYS above 3k RPM and you would never experience the problem.
But it you were tooling around town and eased into the throttle and maybe had a light incline you would experience this chuggle. Down shift 1 gear, engine is above 3k RPM, problem gone, however, if you continued to lay into the throttle the chuggle/bucking would get worse as you approached 2800-3000 RPM. Car would actually loose power.
I was able to find a specific road with a minor incline where I could literally force the problem to occur and occur bad if I was lugging the engine a bit in say 5-6th gear around 2200 RPM.
I finally figured out the issue using a cheap, generic $30 OBD program and I was able to record the ignition timing. The ignition timing was retarding to as low a -5 degrees Before Top Dead Center. This is actually about as low as the timing would go and is really a fail safe value that was used.
The chuggle/misfiring/shuddering I was feeling was not a misfire at all, it was the DME jumping between timing maps reducing power and then when the timing jumps at times as much as 20+ degrees, these were actually power pulses I was feeling when the timing is advancing back from a heavily retarded state.
It was not the DME registering knock sensors and retarding the timing as the knock sensors will only take about 5-6 degrees of timing away, not as I was seeing at times as much as 25-30 degrees of ignition timing being removed. If you take 25-30 degrees of timing advance away in a single instance, you will the engines ability to produce power and it is VERY noticeable.
Very long story short, my issue was resolved with a different DME tune, but it was not a BMW DME tune as I cannot run a stock BMW tune in the car I had issues with.
I speculate to some extent that what I was suffering from was a bad BMW timing map that may have been slightly altered by a tuner at one point that made the issue much worse and more noticeable??
My current tune also cleaned up a lot of other issue like coast down fuel cut off and 2nd gear throttle sensitivity.
My suggestion is to map out your engine RPM, Throttle Position, Ignition Timing and Coolant temp, go for a drive and see if you can catch your problem with the software. If your ignition timing is jumping around a lot, you then need to figure out if you have a bad timing map or a leaning problem causing the knock sensors to retard the timing. But timing retarding should not be really noticeable under most circumstances.
This may not be your issue, but is an easy issue to check and rule either in or out as a culprit.