2000-328i surges when cold

328i w/ 39K idles ok for about 3 minutes at 800 then begins to surge
between 200-1500 about 2-3 times per minute and sometimes stalls. Once warmed to 150ºF idle is ok. This only happens when engine is cold and outdoor temp is below about 55ºF. No idiot lights come on so I didn't
check codes.

Look into replacing your Idle Air Control valve. PM me if you would like a price.

If it was the IAC, shouldn't the idiot light come on?

Not all the time.

With only 39K I'll try cleaning it first & let you know what happens - thanx

my 325 is an 86 so similar issues my not be fixed in the same way (i'm new to this), but mine was surging and hesitating when cold. what worked for mine was getting the valves adjusted and getting the idle position switch replaced.

good luck!