Potterton profile 60e running for only a few miniutes then switches off?

Can any one help me with my heating problems. The heating was working fine with only one radiator downstairs staying cold despite there being no air in it. Now I have only hot radiators upstairs and cold down. The boiler comes on for no more than about 3-4 mins then switches off. I have changed the heating pump, but still the same. All radators have been bleed of air. Any ideas?