About a month ago my car started overheating driving down the freeway. When I pulled over the temp came down and it does not overheat when it idels only when being driven over 80kmp. I also noticed that is is not shifiting into overdirve. I took it to a mechenic and he changed the transmission fluid and filter thinking that would solve the problem. It did not and now the car is acting like it is going to stall out when I am at a stop light. It feels like a little bump in the car and the RPM's dip just a little bit. Any idea what the problem could be?
The idling speed could be too low and I would sugggest cleaning the throttle body and IAC.
As to the overheating at high speeds, if you do not have coolant losses, have the radiator checked. It could be partially clogged resulting in inefficient cooling.