central heating problem old baxi back boiler?

i have a baxi back boiler which is not working properly when i turn on the heating only the upstairs get hot the downstairs get lukewarm the system clicks on and off every 5mins or so and the hot water is fine.this problem has been happening for the past week but today the fuse on the system blew (pump maybe) replaced the fuse and still hot upstairs cold downstairs all radiators are full and bleed.