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			Check the edges of your tires.  This happened to me on a different car, but same symptoms.  Guy at the tire store found a tire was slightly cupped on a edge.  He said when the tire is cold, you drive more on the edges and the issue is more pronounced.  When the tire warms up, it runs more on the center.
		 
				__________________GPRPCA Chief Driving Instructor
 2008 Boxster S Limited Edition  #005
 2008 Cayman S Sport - Signal Green
 1989 928 S4 5 spd - black
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