Problems with Idle from cold start

I own and drive daily a stock 00 GT (except for the K&N air filter). For the last few weeks I have been having a problem with my car idling when I start it, until I drive it for about 15 minutes and get some heat in it. Also, my mileage has gone to crap, 18.5-20.5 mpg on the highway. I had the same problem a few years ago (wouldn't idle from the start, but it was in the summer then), took it to the dealer a couple times (4), didn't have any codes, and finally found out that it was a bad TPS. This time I still have no codes, changed the TPS just in case, and still have the same problem. Has anyone had this problem? What was done to fix it? How can I chack to see if my Temp. Sensor is reading correctly if I am not getting any codes?

~Frustrated
:bang:
 
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