Vaillant turbomax 824e not supplying hot water?

hi

have a trubomax boiler which when running direct hot water on full setting and tap fully open seems to burn intermitanly and eventually locks out giving the f29 code which is insuficcent gas supply, yet if i turn down the temp sensor to half temp. the boiler runs constantly but only supply warm water!! also when using central heating boiler works fine. so im thinking its the gas valve that is faulty any1 else got any ideas. sorry for being long winded!!