Protherm 80e - Heating cuts out before radiators warm?

Hi

Hope somebody can help

I have a Protherm 80e combi boiler. In the last two day's the central heating has only heated the rad's so that they are not cold. Saying they are warm would be stretching the truth. The boiler runs ok but when the display hit's 41 the boiler stops heating. When it drops to 28 the boiler automatically starts heating again.

The problem only applies to the heating. The water from our hot taps is very hot and the boiler automatically comes on when a hot tap is turned on so no problems there

I did clean the secondary heat exchanger which had some dirt (back water) in it which I could hear rattling around before hand. The only difference this made was make the tap water hotter.

Would anybody have any idea what else could be wrong???

Thanks in advance for any suggestions offered