From someone that cooks about a bird a week in all types of weather i'll tell you it's just an air flow thing. The cold bird always bogs down the heat but i never drop that far. I go on about 325 and the pit might drop to 270-280 and rebounds pretty fast.
Increase the size of two of them to 1 inch... How big is your exhaust? If it's to small that will affect temps also.
Mini WSM uds style, Jumbo Mini WSM aka Jimmy, newly renovated Big Poppa Black powder coated UDS, Kettle, Potbelly Stove Smoker Smoke house w/ Smoke Daddy cold smoke generator attached.PitmasterIQ,Guru PartyQ ...Keith