My 2003 Pontiac Sunfire will excellerate.

My 2003 pontiac Sunfire will excellerate unexpectedly while driving or at a stop while in park or netrual to 3000 to 4000 RPm causing the car to surge. It will also fall below 750 RPM while idling and stall at a stop as well. When the car is not doing this it works fine. The problems seem to happen when the car is warmed up and the engine coolant temp is at 90C on the temp guage. Temp guauge stays at normal consistant reading and does not fluctuate. I have changed out the IAC sensor and The Throttle Body Position sensor. I had a mechanic check for codes and there are none. I change the MAP ( Manafold Air Pressure sensor) and the Mass Air flow Sensor on the air intake assemble between the air filter and the Throttle Body. Car is still doing the same thing. These problems are occuring in the winter months with outside tempertures between +10 and -20C.