my glow-worm micron 60ff?

my boiler will fire up at the set time[diggi timer heating only]
but once all the rads[7 doubles] are hot it cuts out and stays off until the house is cold,only then will it fire up again.During this cooling down period the "on" light for the timer stays on.
I recently had the circuit board repaired for the second time in 5 years, I also fitted a new pump to see if it would cure the problem. any ideas??.