Toyota Repair: 99 4runner wont idle when engine hot & cold outside, throttle position sensor, auto tranny

Question
I have a 1999 4runner 4wd with a 4 cylinder engine auto tranny. It runs great 99% of the time. However, everytime the outside temperature gets below around 10 degrees F and the engine gets to full operating temperature (have to drive it to get to the full operationg temp) the engine will no longer idle when you come to stop. The RPM will drop surge a few times and the engine will die. In fact today it was cold enough, around -4 degrees F, that it didn't want to idle very well even before the engine got to full operating temp, didn't die but would idle rough.

I've taken it into several mechanics who mostly wait until the outside air temp is over around 10 degree F to look at it and then can't get it to replicate the problem, and can't get any error codes to show on their diagnostic equipment. One mecahnic, two years ago, thought it was the TPS (throttle position sensor) since their test showed it was a little out of specs. They replaced it but the next winter the same problem again. The most recent mechanic finally replicated the problem (last year) but again determined it was the TPS. He didn't replace it since the TPS was replaced just a year before that. He said to bring it in if it did it again and he would replace the TPS....it did it again but he can't get it in until the weather warms up so no telling if that will fix it or not.

Have you ever seen this problem before and what was the solution? Also I'm getting tired of mechanics not looking at the car when it's performing at its worst....what type of code reader would work with my 4runner? a OBDII? Any information or ideas would be much appriciated....three years of this and I still can't drive it any time the temp is below 10 degrees. Thank You  

Answer
what made them say it was the TPS?
Ive seen them ice up.
Is the hood seal in good shape at the edge of the hood?
If not, it can let air blast the motor while your driving down the road at that low temp and ice up the intake runner.
yes its an OBDII tester youll need.

A good idea would be to take a "snapshot" with the tool and examine the data stored when it runs rough.


Mike