Teledyne spa heater wont fire in damp weather

I have a 10 year old Teledyne Laars Series 2 type ESC heater installed outdoors and used as a spa heater. During dry weather it works great. When it is damp, the pilot ignites and stays lit, but the burner will not fire. There is a buzzing sound like the pilot igniter that continues indefinately. A few hours after the sun comes out, its back to normal. Any suggestions besides moving to a sunny climate?

spa

Loosen and then retighten the screws on the thermostat, or remove the wires and clean the contacts on the thermostat and wire terminals.

Thanks, I'll give it a try this week end and post results.