gas cutting off.alpha eco 240?

Hello all.
when running hot water for a bath, boiler sounds like an aeroplane taking off and landing, continously.Once bath has been run and hot tap turned off,about 5 seconds later the gas safety valve cuts gas off.I have checked and as soon as hot water tap is turned off,gas stops which seems to be working ok.Boiler seems to be overheating for some reason.the heating works well.if any of you lovely people could help.it would be much appreciated.Thanks guy,s.Ps pressure in both boiler and pressure vessel in back of boiler seem ok.