1991 Mazda 626. Mileage: 175,000. I've been having this problem for a few weeks now. My car runs fine until I reach speeds between 55 and 66 mph. The car shakes violently that I feel like I'm going to lose control, as a consequence I can't drive on the freeways at the moment. When I speed up to about 70 mph or stay below 52 mph the car runs fine. What could be the problem? Appreciate the help.