1996 Mercury Mystique, 104K miles. My automatic transmission is acting very strangely; it sporadically shifts into the wrong gears. Sometimes it will downshift just for a moment, then shift back and be fine, and sometimes it will continuously shift into different gears, jerking while it does so. The check engine light isn't on- it did come on for a while, and then when out, but while it was on, I had no problems with the car. Also, the overdrive off light sometimes flashes. I can't find any pattern for when it shifts and when it doesn't, sometimes it's fine for a long time and sometimes it acts up every time I take it out. It does this at speeds ranging from 20 to 70 miles per hour. I've tried putting it into second gear when I'm at low speeds, and that doesn't help. I have also tried turning off overdrive when the light isn't flashing, and that doesn't help either. I have no idea what's going on with it. Any ideas?