I'll start with a little history: 2 years ago, while going up a hill, my GMC Jimmy dropped down to 5 mph and shot up to 5RPMs. Had it towed to a mechanic (or so he claimed!) and they couldn't find anything wrong with it. It has never done that again. Now, if I drive a considerable distance, and then come to a complete stop and then accelerate again, it will go up to about 10mph, jerk harshly, and then continue to accelerate. There is a 'clunk' sound coming from under the truck at the same time that the jerking occurs. The RPMs will go up to 3-3 1/2k, and then after the clunk and the jerk, they will drop back down. Every mechanic I've asked is completely clueless as to what it could be. My service engine soon light comes and goes. When it is present, and it is read, it says 'trans slip at 45 mph' It only does it if I drive a considerable distance (over 20 miles away from home) and only if I go above 50mph. If I stay in town, don't ever go above 50mph...it will never do it. It also doesn't do it all the time. A new problem is that there is a grinding sounds coming from my back end...it almost sounds like I am dragging something behind me, but there is nothing touching the ground. The truck just hit 140,000 miles.