Ariston boiler query?

My hot water pipe coming from my Genus Ariston boiler and leading to my underfloor heating system goes hot for a short burst, then lukewarm again, then hot etc. Whilst this happening the boiler is still fired, it's not cutting out then in again. The hot water for the shower and taps etc is always perfect, no problems there. But it's just the heating circuit, my floor never seems to get any hotter than mildly lukewarm, where-as it used to go nice and warm. Any help would be greatly appreciated.