Problem with Combi boiler and hot water surges?

We have a Worcester 28Si11 combi boiler which works fine EXCEPT THAT we get very erratic flow from the hot tap in the kitchen (not noticeable on other hot taps). Basically, hot water flows after a while when turned on and then slows to a dribble, then stops. If tap is turned a bit more, not much happens and then there is a sudden flow at high pressure. Can't seem to adjust to lower flow and my wife gets sprayed...sounds funny but very annoying. Have tried reducing flow at supply pipe to problem tap but doesn't seem to improve.