Ideal combi boiler - keeps switching off

nozzer35

I have a similar problem to the post by mine, I have a vaillant boiler and had a problem last with and it needed the diverter valve changing, however since this was done the boiler comes on and off loads. When I switch it on it comes up to heat quickly but after about 5 minutes goes off, the radiators are warm but not as hot as they could be, after about 5 minutes max of being off it comes back on again and does this on and off all the time, which surely in thelong run isnt gonig to be good for the boiler. Does anyone know what might be causing this, is it totally unrelated to the valve being changed last week or somehow has the british gas engineer altered something when fitting that. Im stuck as to what it could be and will probably have to get them out again.