This is the second time its done it in the time I have owned the car ( 18 months or so). Its a 1994 Ef i6 Fairmont sedan and has 185,000 on the clock.

Both times its done it is after the car has been parked and engine still cool in temp, gradient of parking area not the issue as first time it was flat and this time it was relativity flat with a slight lean to the right. Get back into the car, start it in park, go to select reverse and you hear a little clunk from around the box and the car rev's like its in neutral. Select drive and it goes forward. Both times its done it I have put the car in neutral and pushed it back a bit, then selected drive and gone forward a small amount then selected reverse and then it works.

Trans fluid is full, it has a trans cooler on it and after this issue pops up the car works fine. Last time it did it was about 5 months ago and it did it only one, today it did it again. I dont know when the last trans service was done but have full intentions in getting one done in the next month or 2. Does it sound like a service issue of something more sinister?
Cheers
Raven

It could have a broken reverse band.

Gab