boiler stops and starts?

Hi I have an ideal ff250 boiler which starts up ok and then stops after a few mins and then restarts a few moments later,and keeps starting and stopping every few mins. If I turn the boiler thermostat down below 6 to 5 the boiler will stop and not run until I turn the boiler stat back up to at least 5.5-6 again. All rads have been bled, all valves on the rads are open, rad stats on max, room stat on max. Can anyone help please.