1995 C4 Vibration

[chucklin]

I have a 1995 LT1 manual 6 speed milage is 102000. New tires are BF Goodrich G Force TA. The car has a Greenwood ground effects kit. At 55-60 MPH it sometimes developes a vibration felt all over the car. It may only do it once a day and may only last for a couple of minutes. It will always go away by itself. While vibrating I have preformed the following tests with out changing the vibration: Pushed in the clutch, kick it out of gear, run 55-60 MPH in 2nd 3rd 4th 5th and 6th, engage the brakes under power, engage the brakes w/o power, rock the car back and forth. Otherwise the preformace is great.
Any ideas?
Chuck in Florida