2001 Jimmy Stalls

My 2001 4X4 Jimmy stalls usually first start of the day. I know it's not looking good when the RPM starts fluctuating from about 900 to 400 at idle. Eventually it drops down to nothing and then stalls. If I keep my foot on the gas the RPMs stay up and it won't stall. Once it's warm it seems okay. Evry time I take it to the dealor, by the time I get there it works fine and they can find no codes. Any Ideas?

Check vacuum lines for leaks, and any mixture control solenoids or idle air control motors for looseness ie they've partially unscrewed, creating a varying air gap.