I have a Ravenheat RSF100ET combi boiler which is about 6 years old. The DHW has been running at inconsistent temperatures for some time now. It will run hot then cool to warm and then hot again then warm and cool. It deos this over a period of 4 minutes or so to a cycle. I did a search on  and i was advised to first replace the potentiometer and if this did not work then descale the heat exchanger. Does this sound reasonable.

Thanks for anyones help.