Agree with Dave - it is probably moisture and/or corrosion on the electrical parts. It sounds as though it lioghts but won't keep running, right? Disconnect the wire from the circuit board to the igniter & flame sensor (all one unit) and reconnect. Maybe clean the terminal with a pencil eraser or electrical contact cleaner. If that doesn't do it, replace the igniter/sensor. They aren't expensive.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL