I had loads of trouble with mine last week. I have put it down to the cold weather.
I use a trolley system that carries 2 x 25 litre containers, all of these containers had ice in with the water.
At these temperatures the water becomes thicker, this in turn requires a bigger draw of amps from the pump. The Varistream works by measuring the amps, so will shut off quicker in these conditions. Ok you can adjust for this, but as the water gets low in the container there is more ice than water, so it gets even thicker.
The problem then arises when you change containers and your running on thinner water, you then have too much pressure and it won't switch off.
I have a smaller pump that works with a 12 volt motor speed controller. Next time I'm working at zero degrees I will use this one instead.
Back packs work fine, and all they have is a device that changes the pump speed.
A speed controller is a lot cheaper than a Varistream too.
I think I paid about £24. just google 12 volt motor speed controller.