I have a ZF 6 speed in my 07' 335i. Have you ever done a service (fluid and filter) on it? My car has right at 100,000 miles on it, and the transmission just felt weird when I first got it. I did a filter and fluid change....and changed the sleeve seals and D seal in in (really easy), and it made a world of difference.
I've heard that these ZF transmissions are very strong...but you have to service them. If you have 177K miles on it, do a fluid and filter/pan change....it will probably cure you your issues, and it sure it cheaper than buying a rebuilt transmission.
