We have that problem one or 2 times a year and not exactly sure why.
We've now learnt (the hard way) that if we turn on the release valve, it releases pressure somewhere in the system and it runs fine again.
Our sytem - 650 litre tank which has purified water (previously held in a 1000 litre tank in our workshop, which goes through a merlin purifier first takes about 10 hours to fill the 1000 litre tank, but only about 6 minutes to transfer to the 650l) - pumped by 12v battery though a 100psi pump, through a DI tank and before the water goes to the 2 sets of hoses & brushes we have a valve which can reduce the pressure if too much water comes out of the brush jets - there never is too much pressure as we have 2 men working off the one pump. When the valve is opened the pressure is released, and the excess water is put back into the tank. Anyway, the valve has also proved useful a couple of times in sorting out the broken pump problem. Before we learnt that we tried changing the battery, cleaning up all electrical points, switches, clearing hoses etc.
May not be relevant, but thought I'd throw in my 2 bob's worth.
Cheers
Rob