my van start sometime, i have replace the alternator, battery and starter and it is still only start sometime

First thing that comes to my mind would be a problem with the alarm system (if equipped)- The second thing I would look into would be the Ignition Switch itself. You need to check and see if the starter is getting 12 volt power at the solenoid (small wire start wire, not the big battery cable), when you are turning the key and the vehicle is not starting - If you are getting 12 volts, more than likely you simply have a faulty starter and you will need to warrantee that one out (it happens all the time!) - If you are not getting 12 volts at the starter when turning the key to the start position, please refer to my first 2 suggestions