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.
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