We have an intermittant fault with our 5/6 year old boiler. The hot water will occasionally run cold after a couple of mins and the boiler shows an S.53 fault code. Turning tap/shower off for a minute or so cures the problem. Water also runs cold if taps turned on high. Engineer came out about 4 weeks ago and was mystifed as although the water was running cold, the boiler was showing that the burner was operating and temperature reading as it should. He phoned Vaillant who said Heat exchanger was blocked. New HE ordered and fitted. No difference. A little scale in the old HE but water was perfectly clean with no sludge or bits. Another call to Vaillant who offered no help. Engineer suggested pump could be at fault as he'd replaced several on this model. Said he would order new one and come back. Not seen him since. Does anyone have any ideas please?