1998 Plymouth Voyager Stalling/Loss of Power

My van has a problem of loosing power and/or stalling. There seems to be no reason for this to be happening. When I am driving at slow speeds, stopped or turning a corner the van will jolt and stall. No lights come on and it has always restarted. When going at faster speeds or travelling down the hiway, it will jolt, but will keep running. This problem has been going on for the last eight months and I have taken it to my mechanic several times. He says that no codes show up when he hooks it up to a computer and has been hesitant to do anything without a clear diagnosis. Last week I ask him to repair what he considered his best guess and he replaced the crankshaft sensor. It seemed to work great for a few days, but it is stalling out again today just like before. My Wife takes my kids out in the van all the time and I want it to run well. Do you have any suggestions to wha the problem might be?
Thanks for letting me know.