Potterton HE30 runs cold after a few minutes?

The boiler has been fine for 3 years but after we turned of the central heating mode the boiler runs cold after 2 or 3 minutes but only when taps or showers some way a way from the boiler are used. The bath which is in the same room does not seem to be affected. The only way to restore the hot water is to turn of the tap and restart after a few seconds.Could this be a water pressure problem ?
the whole house is run through a new modern water softener.