Rough idle during cold weather start

I'm helping a friend with his 2000 Sierra 1500, 5.3L engine. When he starts it in the cold (below 40 degrees) at first hits 1000rpm, then immediately drops to 400-500rpm. The idle is rough and wants to die if he tries to drive it. However after 10-15 minutes and the eng warms up, its fine no problems at all. There is no codes, so the scanner we had didn't tell us anything. He had cleaned the TB and intake system with cleaner, no help. He also disconnected the IAC while it was idling rough, that didn't change the way it ran at all. We would like to toubleshoot it more before we start throwing parts at it. Any one have any suggestions that could help us?

Check the coolant temp sensor, it could be telling the ecm that the engine is warmer than it actually is.