warning light on runs rough then runs fine when light goes off

had car taken apart to fix another problem-week later car started shifting hard and warning light came on(car with wrench). Took back to shop-it stood for about 10 minutes and they then took it for test drive-no light no transmission problems. I drove it for about 2 weeks and same thing happened. I turned off car and then restarted still same problem-drove to shop and again they started it up and no light on and no shifting problem? Right now it is again running fine? what would cause car start shifting hard and warning light on-car with wrench inside and then problem solves itself.