back in the summer my vans water pump started leakingand my van overheated, i replaced the coolant and drove about 15 miles home. i replaced the water pump and thermostat and everthing was fine. a few weeks later i started having a problem with low rpm's and odd shifting of the transmission, when you drive you can be traveling at a steady speed and the rpms will start to increase but the van's speed won't, you can increase the gas the rpms will increase like the van is in netural. when the rpms increase the van learch forward and then will upshift, then down shift. this goes on over an over. if it is cool out side or first thing in the morning or raining it operates normally, or if you drive 75 mph or higher it operates normally. the only code shown is random missfire and this code has only shown up one time, and on the gm techs computer the transmission shows fine, but he thinks my throttle body needs to b replaced. i replaced the throttle possition sensor but no real change. i was told the throttle body needed cleaning, i cleaned it and the mass air sensor but again no change. any suggestions?