I have an 83 633csi, I noticed I had problems shifting fron 2nd to third. Then one day it became difficult to shift at all, and eventually stopped shifting. It turned out the throw out bearing siezed up. This might be your problem, but I'm no mechanic-Justin

My '82 633 won't shift out of first for the first mile when cold or after sitting for more than 2 hours. After that it works fine except for sometimes not shifting down from 2nd to first when it should. Any ideas?
Thanks, Mike

I went with synthetic gear oil and that solved my problem