Intermittent Fault Vaillant turbomax VUW 242E?

How can I fix my Vaillant Turbomax VUW 242E?

My boiler is 8 yrs old. The flame indicator illuminates but then after about 15 seconds the burner lock out indicator goes out and so both the central heating and hot water malfunctions intermittently. Therefore the central heating radiators are not getting hot enough and the hot water runs cold then hot then cold again etc. I would be grateful for any advice/or suggestions as to what could be causing this problem, and any possible solutions.

Mr and Mrs Henot- Sheffield
January 2007