intermittent vibration

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I bought a used 2010 yukon xl with 16500 miles from dealer. the vehicle is certified, my problem is a get a quick vibration in front wheels and steering wheel when vehicle is going between 38 and 40 mph....this only happens when vehicle is first driven in morning or sits for a while...when the vibration happens it is about every 100ft unless i go above 40mph...dealer cannot find anything wrong, and by the time i get it to the dealer the vehicle has been driven about 10 miles and vibration stops....vibration only lasts a few seconds...please help