Well, I had the same problem on my 95 mystique. Mostly when driving on the freeway, it would all of a sudden start dropping into 2nd gear then back and forth between 2nd and drive. It drove me crazy for a while. I first tried a fluid and filter change. That helped but then the problem came back. Then it seemed more when the engine was warm so i put in a lower temp thermostat. Again, that helped but the problem stil came back.
So eventually it gave me some computer codes even though the check engine light never came on. The codes pointed me to the TR Sensor out of range. The Transmission Range Sensor is a switch that sits on top of the transmission that gives a particular resistance value to the computer depending on what gear the transmission is in. When it goes out of range, the computer defaults to 2nd gear. Worst of all is when its intermittant, it will go out of range and drop into 2nd gear and then come back and drop back into drive and can do this rapidly. Personally, I think this i a crappy design by Ford. Possible codes: 522,634,659,667,668,675.
Anyhow, now I have a 96 Windstar (got a real good deal on it) and after having it a couple months its pulling the same crap.
Now the good stuff... its been a while, so my notes from a couple of years ago might not be 100% accurate, but here goes...
How to test the TR sensor:
1) Secure vehicle with emergency brake or block tires
2) With engine off, place vehicle in drive
3) Verify alignment mark is aligned with 'D' mark on TR sensor. If not aligned, adjust shift linkage. If 'D' mark is alignined, verify sensor voltage and resistance in following steps
4) Disconect TR sensor, turn ingnition on and check voltage between pin 7 (red/black) and pin 1 (black/blue). If voltage is between 4.75-5.25 volts procede to next step. If not, repair damaged connection
5) Measure resistance between pin 7 and pin 1 (here is where my notes were not clear, but I believe these are the correct pins). Resistance should be as follows (in ohms)
P 3770-4607 ohms
R 1304-1593
N 660-807
D 361-442
2 190-232
1 78-95
Out of range, replace sensor. Intermittant problem you might want to replace anyhow.
