The exact same problem as on this page:
I recently bought a 1998 Ford Taurus with the Duratec 200 h.p. engine. As I was driving the " overdrive " light started flashing, and the speedometer dropped to "zero" for a few seconds and then started working intermittently. The transmission then felt like it slipped into "neutral" for a few seconds and revved up to 3500 rpm's. A second or so later the car slipped back into gear. I brought the car to my mechanic and he said the problem was caused by a faulty speed sensor. He replaced the sensor and for a few hours everything was fine. The next day the same problem with my speedometer and transmission started all over again, only this time the "overdrive " light did not flash. I went back to my mechanic and he said nothing was showing up on his diagnostic test. He referred me to a transmission specialist who also ran diagnostics and said nothing was showing up. He suggested that maybe the problem was that the transmission was heating up due to a clogged or cracked transmission filter and that I should replace both , even though both mechanics said the fluid looked fine. Desperate, and running out of money, I changed the transmission fluid and filter . The same problems still kept happening with the transmission and speedometer. About a week later , the "overdrive " light started blinking along with the transmission and speedometer problems, only this time the "overdrive " light would only stay on for about a minute and then turn off : however the transmission continues to feel like it is slipping for a few seconds as it revs up to about 3500 r.p.m.s and the speedometer fails for a short time over and over.
Thursday, January 9th, 2014 AT 8:22 AM