van will shake

I have a 93 chevy lumina van. It runs good when it's cold. After it gets warmed up it will run ok till about 50/55 miles per hr. Then it will start jerking and shakes really bad. At first it seemed like it was missing but now it's more like a jerk. It does this intermitintly. but seems to be doing it more often and more severe. If I step on the gas it will come out of it till i slow down to that speed again. Also likes to do it at a slow speed like 35/40. The other night it did it just as i was stopping for a stop sign. When i went to go, it would kill. About the fifth try it took off. When I drive 65 and above it runs good.
We replaced the wires, spark plugs, cap, distributor, Put in an ignition sensor.and processor, and checked the distributor shaft. What is left to try? Could it be a coil or egr valve? Or what? We are stumped. Any help would be appreciated. Thanks, Andie

It may be a sticking torque converter lockup solenoid. YOu can unplug it and see if the problem goes away. To do so, look for a square plug on the side of the transmission (side towards the radiator) and unplug it. Then see if the problem goes away. You can leave it unplugged if you don't want to get it fixed (could cost $$$$), but you would notice a 2-3 mile per gallon drop in your fuel mileage. This is a common problem on GM vehicles late 80's to mid 90's.

[QUOTE=mlw35405]It may be a sticking torque converter lockup solenoid. YOu can unplug it and see if the problem goes away. To do so, look for a square plug on the side of the transmission (side towards the radiator) and unplug it. Then see if the problem goes away. You can leave it unplugged if you don't want to get it fixed (could cost $$$$), but you would notice a 2-3 mile per gallon drop in your fuel mileage. This is a common problem on GM vehicles late 80's to mid 90's.[/QUOTe

Thank you so much. We unplugged the torque converter and drove it about 30 miles, never once did it shake or jerk. I think the problem is solved. I really appreciate your help. This is a great forum. Thanks again, Andie