Why does the motor pulsate when trigger is released

Ron, I do not see a big issue here. The trigger mechanism on your handle simply cuts off the water supply. Yes, your pump will continue to do its job until a pressure switch turn it off. Since you have a small leak somewhere it is possible that the pump does not reach it's cutoff pressure. It might be possible to drill down to the issues but for that you would have to at least let us know what make and model your pressure washer is.