: The engine of my '91 850Ci has recently started
: to vibrate rather heavily when cold - as if there was
: some kind of unbalance or poor timing. Typically it
: does this for about one minute and then it starts
: running just fine (sometimes with a rumbling noise
: just before getting well).

Whoooa, just found the problem. Several blades in of the cooler fan are missing. No wonder there is a bit of unbalance. Not to mention that rumbling noise - probably when another one of the blades took off - temporarily leaving it in some more balanced state. I'm off to the shop...