Potterton Puma 80e hot water very hot then runs cold

[awksed]

Hi All. I have a Puma 80e which recently has started to deliver very hot water (hotter than before), it does this for perhaps a minute and then the hot switches off and you get cold water for about a minute or so (the flames stop burning) and then the water starts getting hot again until its really hot and then it goes cold again etc and just fluctuates between too hot and cold. I haven't tried it with the CH running as some posts have suggested trying. Any ideas anybody? Exorcism?
Thanks in advance.

 

[twgas]

Does the flow rate increase a few seconds after the heated water passes through the boiler?

 

[ks52]

Rgi required. Need to know if modulating which i suspect not. Could be one of a few offenders but only way to find out is with the right equipment and knowledge....