My overdrive seems to be engaging without being asked to. I noticed it first driving in 3rd gear, the car seemed to feel like it was slipping into O.D. I checked the switch; it was not activated and the OD light was not on. When I shifted to 4th, it engaged when I moved the switch and the light came on. I took it out of O.D., could feel it disengage and the light went off. I drove around a bit more and again felt an RPM drop in various gears like the OD was engaging. But the light was off and the switch untouched. Then, after stopping and putting the car into reverse, it shimmied and jumped. I can only suspect that the O.D. was engaged. I quickly shifted to first, U-turned, pulled it into the garage and parked it. Any suggestions for the next thing to try?