Erratic Idle when cold

Hi.. I own a 2003 Ford F-250 Truck, with a 6.0 Turbo Diesel Engine. It's a 4 x 4 with approx 92000 miles. When my truck is first started it idles normally for a minute, and then the idle speed jumps up instantly to around 1100 rpm (erratically). The check engine light is on, and remains on. Any ideas as to the problem?? When the truck is warmed up it idles normally, and no degradation of performance has been noticed..
Thank you very much.
Darrel