Why does the water go from hot to cold every few seconds?

under the boiler on the far right hand side is a black headed tap on the cold inlet to the boiler this is cold water entering the boiler, with a hot tap open, boiler switched off, turn it off bit by bit til the hot water flow from the tap gets down to a slower rate about half of what it was. turn the boiler back on and see if it stays hot now and report back.
not a fix just wanna know outcome to guide

ste
December 2009