running rough in cold temperature

I have a 2001 ZTS with 141k miles on it. This is the first winter I've noticed this condition and its driving me nuts trying to diagnose
it. When the outside temperature (New England) is cold the vehicle runs horrible. It sounds almost like an exhaust leak and is accompained by severe vibrations. No stalling or hesitation but remains consistant as long as it is cold out. When the outside temperature warms a little the car runs nice and smooth. Does or did anyone experience the same problem? Thoughts?
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