Phantom starts?

Combi 80 Central heating off, summer use of hot water only. Boiler static,power on, when extract fan starts short gas burning period, then flame cuts out but fan continues to run. The fan stops after an extended period of running without intervention. If the hot tap is opened after such an incident the water is almost boiling. Central heating switched off. Switch off boiler and fan stops, switch on the boiler and the fan starts. Lot of head scratching!