Sounds like it could be a dirty or faulty idle air control valve.
I've had the vacuum advance inside the distributor decide to randomly adjust the spark...that might be something to look at...vacuum and centrifugal advance...not certain that yours still has this technology...but I'd look at the vacuum advance~ vacuum lines degrade and leak...if only they were not so cheap to use the butyl rubber...BMW always uses nitrile rubber on all their vacuum lines...verrry suspicious~
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