the boiler now lights up and cuts of after 30 seconds?

(hi hope you can help with my worcester 24cdi boiler problem
my heating is working up stairs rads get very hot .
but they do not get hot downstairs at all.
i have changed the rads and new valves on all ,

have bled the system a few times now ,
and no air is in the system)

i followed mr gas advice (thank you ) and have got it down to two rads that are not getting hot properly .

the boiler now lights up and cuts of after 30 seconds and then comes back on after about 1 minute and runs for 30 seconds and then cuts out again .
do have a slight leak on the heat exchanger very small drip and i mean small .
any more ideas please as my head now hurts lol