1998 Pontiac Grand Prix Front Tires Feel Flat Cr Wont Over 45 Miles Per Hour

Have you checked tire pressure? As far as the are poor running, if the check engine light is on, have someone scan the computer for trouble codes. Most will do it for free.

I would also advice you to check the tire pressure. if its okay and this problem persists, might be something wrong with the tube. The tube might be made of a low quality material that gets heated up and overstretches when the wheels are in contact with the ground. As the speed increases, so does the overheating and this causes the wheels not to move.
If you are not sure, just visit the nearest garage for a mechanical check up.