Fast idle

I have a 1999 ford escort 2.0 spi engine, manual transmission with about 160,000 miles on it. I live in minnesota, and when winter rolls around, my engine doesnt want to idle down. It usually has to get down into the 40's, before it happens. Runs perfect in the summer. When I start the motor up cold it idles fine, but as soon as I start driving, it stays at a fast idle. As long as the car is moving, its stays at a really high idle, but about 3 or 4 seconds after I stop, it will idle down some but be higher than it should. I get a p1506 code for the iac overspeed error after idling for about 20 seconds. I replaced the iac but it didnt change anything. I also replaced the air intake temp. Sensor, which had no effect. I then cleaned the throttle body, and now it stays at an even higher idle. The past 3 or 4 winters it would ilde down some while driving, but now I can take it out of gear doing 60 mph (5th gear) and let it coast, and the engine speed will increase to a slightly higher rpm and hold there untill I come to a stop. If I unplug the iac, it will idle like it should, plug it back in and it idles high. So the computer is telling it to idle high, and also has something to do with the colder air temp. I'm running out of options and really do not want to bring it in. Any suggestions on what it could be would be greatly appreciated.