I know my 89 gt was the same way. At 100 mph the car felt loose and seemed to float at that speed.
My 85 svo is the exact opposite at 100mph your a stable as can be. Since the svo is a fox just like the gt the main difference I would bet is in the suspension.

If you're going to be doing alot of higher speed driving then drop it a few inches with some better shocks/struts and you will be set.