Diagnosis and cure for a baxi bahama 100?

When my Baxi Bahama 100 is switched on the pump starts and the orange ignition light comes on. About 7 seconds later the gas ignites anoither ten seconds later the burner level rises. At this time there is a hissing noise that continues for about 30 seconds. Then the boiler makes its normal running noise but swithes off after another within the next thirty seconds. It will continue this cycle for as long as I am prepared to play with it. Can anyone advise other than throwing it away?