Help: Idle Surge During Cold Weather Only

I own a 99 328i sedan. Last winter, it started an odd idle surge that
happens while stopped at a signal, but only when outside temperature is
below say 35F. Once the car is warmed up and idling normally, all of a
sudden, the idle speed just bumps to approx 1500 or so and comes back within
1 second. This stopped since the end of last winter and just started up
again (Detroit!).
I tried looking up on the web, but didn't find anything specific to cold
weather idle surge, although idle surge seems to be a common issue with
bimmers. I would appreciate any pointers - don't want to take it to the
dealer and get fleeced.... Thanks in advance. Any help would be
appreciated.