My 2001 Hyundai accent GL is having problems shifting into over drive. I have to drive the car for about 30 mins before it will shift into over drive. After that, the car runs and shifts fine for the rest if the day. I took it over to Aamco and they told me that they thought it was the temperature sensor in the transmission. I think I'm being taken.... Does anybody have any ideas what it could be??? PLEASE HELP