I would think that if you are starving a pressure washer, it would make air bubbles.
If I don't purge the water hose before starting the washer, it stops until the air is worked out of the pump.
Every 28 inches of water column will make 1 psi. You may need to raise your tank. Can you add air pressure to your storage tank?
I like using rotary nozzles. Have you tried them?
I never have less than 20 psi going to my pressure washer, so I can't say what happens with low pressure going to the pump.
Does your 1/2 inch pipe deliver 3 gallons per minute? Most pressure washers use less than that, 2 + gallons/min. Try increasing the pipe size.
Take a pump and boost the water pressure before goes to the pressure washer and see what happens.