When I start my M42 powered 1994 E36 318is the revs jump up to about 1500 then drop down to almost stalling speeds, then go up and down a few more times, then stabilise at normal idle speed, about 800rpm.
I have recorded a video (no audio) of a typical start.
This happens whether the engine is cold or hot and once running there are no idle or running problems whatsoever.
The problem is somewhat intermittent. It happens most of the time but not always.