how to locate problem?

baxi 105 he hot water ok .when heating selected low pressure punp fault led lights if you run hot water 5 seconds then turn off. boiler stays lit and leds all work ok and rads heat up but when temp is reached boiler turns of and sometimes does not relight. turn water on again and all ok. have fitted new diaphram pressure set 2bar runs 4/5 days then same problem