CO Detectors use a PPM (Parts per Million) vs. Time, once it gets to a minimum level over a certain period of time, the detector will go off.
I would suspect a backdraft if it is extrememly windy.
Check your chimney for obstructions, bee's nest, etc...check all your exhust joints and put new sealant on.
Our CO did the same thing, but had it plugged into the same outlet as our humidifier and when it cycled on/off, it set off the CO. I moved it and works fine now.
We have direct vent on our stove, so it is alway drawing a vaccum in the stove via the exhaust fan. You Always need to check the draft every year to make sure.