My Ideal Isar HE35 gone cold...help?

I have an Ideal Isar HE35 combi boiler. When running a bath or shower the temperature goes from hot to cold and back again every 10 seconds or so. The pressure is not affected. There seems to be enough hot water though for small things like hand washing or washing up.

When running a bath or shower the burner light goes on then off and on again which i imagine is when the temperature goes hot - cold - hot, but i don't know why. When not in use it displays 'o' when in use it displays 'd'. Can anyone hep me as to what might be wrong?

The boiler is only two years old although it hasn't been serviced yet.