My 850 has less than 35000 miles and experienced some intermittent slipping when it was placed into "D" after being in P/R/N, when it was warm.

I went to 2 transmission shops who said the fluid was fine (though brown) and the car's transmission was going to need total overhaul/replacement, of course at exorbitant cost. I went to a volvo specialist who said that the gear position selector switch was failing, and they had never seen any internal mechanical trouble with an 850 tranny but had replaced lots of gear position selector switches. 3 places, 3 opinions.

I followed the good advice of respondents on this board and am now happy to say that after filling the transmission with new, standard Dexron III, there is no more problem. I changed it myself and followed the volvo procedure for a total ATF flush, but merely took the top hose out of the cooler and drained all the fluid that way. I only needed about 11 litres of ATF for the total flush.

Thanks to all of you who responded to me, and I hope that anyone who has tranny problems can fix them with this simple solution.