Hood Ajar sensor comes on and off

I have a Pontiac Bonneville 97 with 77,000 miles on it. There is this very annoying problem with the Hood Ajar sensor. It beeps and the light turns on, and then immediately turns off, then beeps and turns on, etc. It keeps repeating, turning itself off and on and making the loud beeps. My hood is not ajar. For some reason this only happens in the winter months when it is cold outside, and the rest of the year it is fine. Rain and/or bumpy roads make it worse. It is an intermittant problem but seems like it gets worse each winter. Any ideas of what I can do to fix it? Anybody else have stuck sensor issues?

Thanks.

Spring.