Q: Car independently accelerated to high speed into intersection

Before entering a main road coming from a parking lot area, I stopped at a stop sign. My car started independently accelerating to a high speed which caused my car to move toward the intersection. Fortunately no cars were approaching so I kept turning, but it was a very unsafe situation. This had happened before in town and also in my neighborhood. The conditions each time were the same: while beginning to accelerate my car would jump to a high gear or speed. It was on a high rev idle this morning and did not go down until I changed to manual and then back to automatic. What should I do to get my car back in good performing condition?