I have the same problem if I turn the dial to max. I only use it in the 9oclock position, that's fine for me. I'm using 60psi with 6mm Too. I only have 30 mtrs on the reel.
Are you using 6mm up the pole too? You may have a problem if you reduce the diametre of your pole hose.
The water needs to flow with as few restrictions as possible. If you get a build up of pressure and it exceeds the cut off setting, the Varistream will sense it an cut off the power to the pump. You could try adjusting the dial inside the Varistream. If you turn it up a notch it will then cut off at a higher pressure. Worth a try, Dai
