Potterton Promax HE Plus Hot Water Problem

[higgsboyo]

Hi all, I have looked through this site to see if anyone has had anything similar but couldnt find anything.
Anyway I have a Potterton Promax HE Plus which has recently been struggling to provide hot water. When you turn the hot water on full the temperature on the unit rises rapidly to the high 90's and then loud bangs occur which causes the pressure indicator to fluctuate up and down wildly (which is odd as I didnt think this had anything to do with water flow) and the hot water flame indicator to go off until the indicated temperature drops to around 60 degrees (which it does rapidly) and then the cycle is repeated.
A plumber (who refitted our system after building work was completed) has spoken to Potterton and replaced one of the sensors (dont know which one at present) but the problem remains. Interestingly, if you run the hot water at the tap at half pressure then it runs more or less fine stabilising the hot water at around 64 degrees (even though the temperature on the wheel setting is 50 degrees).
Has anyone any ideas on what could be the route cause here?

 

[45yearsagasman]

That does sound like a restricted flow through the plate heat exchanger. I'd be removing that first and check for blockages in the primary side.