What color is the car???? I have had severe problems with some blue ones!!!!!

Seriously====many of the vehicles built before the late 90s become an nightmare at high speeds, but us racers are trying to drive them much faster than they ran originally----on our Pinto ( someone actually put a pinto in a wind tunnel and tested both directions---they were far better backwards!!!!!) we cut out the tail lights and lower part of rear window to vent pressure from under rear of car--we also put louvers over the rest of the rear window area and vented between them also( should of pattened the louvers, they got copied for the aftermarket)and changed the rear spoiler quite a bit to sorta resemble the trunk lids/rear of other cars at the time ( Mopar 'cudas, etc)
all this helped, but then the car went into giant wheel stand (pics around here somewhere) Built a Mustang, problem solved---

On my Monza pro stocker--sorta same scene---changed springs, schock settings, wheelie bars----made a different spoiler and it became one of the first 7 second cars--didn't have handling problem until about 140 but after that----- with the spoiler change 180 plus---

Keep in mind that most airplanes will stay airborn down to around 120 so aero at that speed is pretty signicant---course that was in a different lifetime also

As to the color comment---put some brightly colored yarn on the car and photo graft it at speed ( can be very enlightening as slow as 40-50) to see where the air is going and in what direction