Car jerks on downshift

2002 dodge intrepid. Not sure on engine size (something like 3.4L). When slowing to stop at a light or stop sign, car jerks when shifting into the low gear at around 15 mph. Then once I begin to accelerate again, wont shift from third to fourth. The engine rpm's go really high. I have found two solutions for the time. One - shift to neutral before I get below 20 mph. Two - pull over, shut the car off for a second and start back up. This allows car to shift to 4th gear but goes back to same once slowing down again. Dont know if this is a transmission or computer or what kind of problem.