AO Smith Sealed Shot water heater problem.

[Woodcop]

I have a AO Smith sealed shot power vented gas water heater that was installed in 1998. [black]Twice[/black] this year it has needed repair. The burner lights (theres no pilot light on this model) and then within about 2 or 3 seconds it shots off. It was repaired once (I don't know what they replaced) and now it's doing it again. Any suggestions? When I built the house in 1985 it had a cheap water heater that lasted until 1998. I paid big bucks for this model and the %$?#@&* thing keeps breaking down. Thanks in advance.