Car won't start

When it's very cold in the garage, my Jeep starts right off. The more I drive it, the longer the cranking time gets. Last Fall, Chrysler paid to have a full diagnostics run on it and they could not find the problem .We have 2 other Jeeps that do the same thing. Chrysler say they have no record of this problem from anyone else. I told them that all 3 of our Jeeps have this problem and not anyone else in the US does? The mechanic told me the only thing he can think of is to put on a different throttle body. They're about $250.00 new. He told me to get one from a salvage yard and he also told me how to clean it first. I haven't done that yet but I will when the weather warms up. I hope you find your answer . If you do, please share it here. Best of luck. My Jeep is a 2004 Grand.