As already said, check fluid level, if ok, and it's been a while since you changed the fliud and filter, thats the next thing to try, if that doesn't fix it theres an internal problem. I've had a trans fail like that before in the past, suddenly just had no grab in one gear, it turned out the lip seal on the clutch pack servo had gotten hard with age and a section of it had chipped out, so it couldnt make enough pressure on the clutches for a good hold. If that is the problem and it is not corrected soon, it will burn up the clutches and possibly some other parts slipping like that.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale