What is wrong with my pressure washer?

My 6.5 HP Craftsman Pressure Washer runs great on th engine side of things. It delivers great pressure and is a great tool. However, sometimes (and becoming more regular then it used to) when I release the trigger on the spray wand it won't spray again when I pull the trigger. The engine still runs fine when this happens. The ONLY way to get it working again is to shut down the engine, turn off the water and disconnect the garden hose AND release the pressure hose from either the wand or the pump end (have to use channel lock pliers to release the pressure). What is wrong? Is it the wand? Is it something in the pump? I would think there would be some sort of pressure relief valve in the system somewhere. Basically, once I start washing, I can't stop until I am finished or I have to turn off the engine and let the pressure die down before releasing the wand trigger. HELP !!!!!!