Have you checked for trouble codes?
From your description of the symptoms, I'm tempted to think your IAT (intake air temperature) sensor may be defective. It's located either in your intake manifold or it's built in your MAP sensor (which is located in your intake manifold as well). Basically your computer relies on this sensor's input to know the air density and changes the air/fuel ratio consequently.
Have your mechanic check that. If you're mechanically inclined and you have a scan tool or a DVOM you can check this sensor yourself.
Let me know
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Thursday, November 5th, 2009 AT 4:32 PM