Ferroli combi boiler problem?

Hi, my Ferroli Domi compact F24B boiler has a problem with the hot water output. The water gets up to temp, but as soon as it's there, the burner turns off, causing the water to go instantly cold, then it has to heat it all over again. When filling the bath, it does this several times, every 30 sec or so, and end up with a mildly warm bath. Very annoying!