This is a condition which is considered and emergency and you should take or tow the vehicle to a repair shop immediately. The transmission is on its last leg and will pick the most inopportune moment to strand you or cause a horrific accident. This is not a DIY job.
we would need more info on this question there a few reasons this could happen depending on the circumstances ie bad linkage detents or loose linkage internal transmission problems ect.
Agreed, but for those who have gotten thier cars with high mileage already put onto them, I'd say it's best not to chance it unless absolutely necesary.
Check the easy possibilities: first look to see if your transmission fluid is low. Then check to make sure the transmission isn't leaking. Finally, check if your filter is clogged.