We have this 2000 Venture van. The transmission seems to jerk into each gear, as well we hear this faint whining sound in first gear as it approaches the second gear. It seems to happen only in hotter weather after about 30 minutes of driving. When the temperature is below 20 degrees Celsius, we could drive for hours and hours without anything happening. We brought it into a mechanic and they can't find anything wrong with it. Is there something we could do? It should be good for the winter as it gets to -20 on a average winter day here in Winnipeg so it shouldn't be a problem, at least until next summer.