Trouble with Hot Water?

I have a Worchester 9.24 BF Combi and had very little or no trouble with it over the past few years. However on running a shower or hot water tap after about 5 minutes the boiler and vent pipe starts to shake and bang with the vent pipe getting extremely hot, It sounds like the boiler could blow up at any point. The water then runs cold. The central heating system has no problems whatsoever. Please can anyone advise as to what the problem maybe. Thank you and regards.