My nearly-new Kaercher pressure washer has developed a problem... when
I use it with the 'normal' wand (as opposed to the twirly dirt-blaster
thingy), the motor/pump keeps cutting in and out so that I get a
pulsatile flow of about two blasts per second. (I have occasionally
seen this with the twirly wand as well).

What could be causing this? I wondered about a partial blockage in
the wand - would that cause these symptoms? It seems to work
perfectly well when the pump is in its 'on' phase, and I can't see
anything amiss. The water supply by hose is fine I'm sure - it's fed
from a direct mains-fed tap.

Any advice ?
Thanks
David