Tim you need to recirculate/bypass some of your water,
sounds like your pump is pumping more water than you are using so every now and then it stops to let you catch up/lower the pressure, then starts again.
Get a hozelock multi connector and a length of hose, drill a hole in the lid of your tank and connect hose to multi and feed into hole in lid, then turn on pump with pole attached and start opening valve on connector to extra bit of hose until desired flow at the brush is acheived, you will now have water recirculating back into tank and pump will not stop.
Only problem with this is that it uses more power.
Steve
see attached pic of mine.