94 Gr. Caravan bucking, shaking problem

Having a problem w/our van "bucking" or shaking, even in neutral. Been told it was the oxygen sensor and MAP sensor, but replacing them did not fix the problem. When driving and this problem starts, it feels like it's just going to cut off. One place has told us they don't know what's wrong, and now it seems that the dealer is guessing at the problem. Any ideas would be helpful.