Ex-cell 1300 psi washer pulses on pressure wash?

The machine was working normal until one time I shut it down. A few minutes later I started to use it and it would pulse about every few seconds. I check the water to be sure it had enough, it was OK. Runs fine for soaping, no pulsing. Took nozzle end off and it did not pulse. Had a good stream of water. Checked to be sure nothing was in the wand tube, clear. It has a push/pull wand. Could there be something wrong with that.