Have the trans fluid level checked, this can only be done from underneath the car. If low, top off and see if behavior changes. If its not low you can try a fluid change and filter/pan change, but it's no guarantee you won't need a new trans. My guess is hoepwever it may just be low, hope that's the case for you.

The clunk is most likely your guibo joint, at that mileage its definitely worn out and ready for replacement.

Most likely the oil level in the tranny is low. It stops working in order to protect the tranny of any further damage.

Normally, the transmission would go to failsafe and it would show on the dash that is in failsafe and that you should contact the dealer. If that's the case, then it definitely your fluid level. I had this issue and I changed the tranny pan/filter as well as the fluid (it took 8 quarts). While I was under the car, I also changed the guibo joint. My shifting feels better and I don't have the hard shift anymore. Bmw wanted me to replace the transmission and I took the car to my house and changed he pan as well as the fluid and the guibo MYSELF. I've just got upgraded from an A accountant to a B Mechanic!

Hey guys, believe it or not, it could also be that the Alternator (Generator) is going bad. The same thing happened to me as I was driving the my 2004 745LI, the Transmission went to failsafe, I could only drive 15 mile per hour and the transmission did not shift. I took it to the mechanic and it was the problem was the alternator; I changed it and never had this problem ever since. That was about a year ago.