fill the system up to 1.5 bar and bleed all radiators from the top valve

how long does the boiler run for before it switches off?

do you thermostatic radiator valves? if so check that the pins inside are not stuck down after being off for a while

could also be a diverter valve or pump fault if none of the above.

really need to be in front of it so i can see what its doing first hand