This is really a tool more than an appliance, you might get more and better answers in the Home Repair Forum than here. However, I will take a stab at it:

I have a gas pressure washer. It uses a pressure sensor on the output side of the pump to determine whether the wand handle is actuated. When I pull the handle, the pressure drops on the output side. This forces some kind of valve, likely a ball held in place by a spring, to stop bypassing the pump and allow pressure to build up on the outlet side again. You can hear the engine load up when the pump is engaged.

What I would look for is some kind of stuck or broken pressure sensor on your unit. If it has an adjustable output pressure knob, look at the mechanism that knob controls. Make sure it is clean, not broken, not corroded. You do not mention which model you own, but if you look that up, there are all kinds of exploded parts diagrams available through the Googlie thing.

You might also search around other forums on the web outside GW for people with the same symptoms your washer is exhibiting, but I am guessing it is something gunked up inside that just needs to be cleaned and serviced. Best of luck.