GE washer will not agitate,will spin

My GE top load washer will not agitate. It will spin in the rinse cycle.
I have already replaced the belt, bell coupler and agitator. It starts to agitate in the wash cycle but suddenly the agitator stops moving. Could this problem be the transmission? or could it be the motor? Honestly this is driving me crazy Help!! the reviews for new washers are so good. I have had to replace the belt on this machine 3 times, could it be that the pulley system is positioned wrong?:eek: