Boiler has intermittent fault with firing up?

Have baxi solo 3 pfl 40 gas boiler. It powers up fine inthe morning but after a while it goes off. Boiler light on also fan but not pilot or burner. Have to switch the whole system off for about an hour then switch it on again and it powers up. Have new room stat and pump, both of which are working OK and pump continues to pump water even when it cools down.