Strong pressure fluctuations when nozzle NOT blocked?

I have a Challenge Xtreme 2200W Pressure washer and was using it normally (31 Jan 2010) when the motor started to 'hunt' and the spray came in bursts - making it not useable. I have checked every bit of the water flow - filter, pipes, lance and nozzle and there are no obstructions. Has anybody any ideas why it should suddenly develop such a fault?