how to fix this worcester 24cdi boiler?

Ok this is the problem ,
rads upstairs get very hot,
ones downstars do not get hot,
i have left heating on for hours ,
have changed rads and valves ,
i put the heating on and waited for the upstairs rads to heat up,
i then shut the valves off both sides on them ,

but then the boiler fires up and cuts off after 30 seconds ,
it then comes on again about 1 min later and fires up for 30 seconds then shuts off again and so on and so on ,
i do have a small and i mean very small drip from the heat exchanger ,
but if i reopen the valves on the rads upstairs the boiler cuts back in and runs until the rads upstairs are nice and hot then cuts out ,
but still the rads downstair do not get hot ?????

the hot water is working ok we do not have any problems with that ,

Brian R
September 2007