Intrmitent hot water whilst running the shower or taps?

I have worcester 28i RSF combi boiler,
Approx 7 years old,
The hot water cycles hot and cold at about 30 second to 1 min intervals whilst using the shower or hot water tap. This fault has been intermitent on previous occassions but now appears to be here to stay.
On the odd occassion it will maintain temperature after cycling 6 or7 times but is to hot to stand under. if I then add cold water the hot / cold cycle resumes.

Nigel
January 2009