Why won't my 2005 Chrysler Town and Country run unlessit is fully warmed up?

We have a '05 Town and Country with 85,000 miles on it. Florida car. Runs great when the weather is warm. During the winters where we are the temp in the morning gets into the 40's to the low 50's. When it is cold it will crank and start up but it will buck and stall if you don't keep your foot on the gas until it fully warms up (to about halfway on the temp gauge). If you try to drive it before fully warm it will keep bucking and stalling out until you park it and keep gassing it. Usually takes 10-20 minutes to get it to temp then it's fine. We have replaced the catalytic converter, map and O2 sensors. The mechanics cannot figure out the problem. They said the fuel system is fine. It won't throw a code. I don't know what else to do