you hav an air flow problem, either weak fan remove fan clean and lube make sure venturis are clear from dust and debri and are not damaged common fault, check pressure tubes for blockages and damage and check air pressure switch diaphram switch operation refit fan and tubes and test check suction of fan with digital manometer and compare 2 air pressure switch requirements, check u have correct voltage 2 fan and ignition pressure is correct also check min and max burner pressures
blaiross, December 2011