Bruce, the vibration you feel after your car sits for a while in the cold is likely due to a temporary set of the tires (flat spot on bottom) that goes away as the tires warm up. Try having you wheels/tires road force balanced with a Hunter balancing machine to remove the last noticeable bit of vibration. Otherwise you may have a tire problem ... slipped belt, etc. Perhaps you could mount another set of wheels/tires to see if the vibration disappears. I presume all suspension components are up to snuff.

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Fred Hoelzle