I just finished repairing the transmission on my '98 328iC. It suffered from the infamous 'fifth gear lean' condition. The M3 used the same transmission and I will be doing this repair to my '98 M3 very soon.
The problem usually starts with the shift lever sticking to the right. Sometimes it doesn't want to pop back to center like it should. It can also stick towards reverse. Another related symptom is difficulty getting into gears 1-4 ('notchy').
I had the transmission out to put in a new clutch anyway so I bought the tools ($$$) to replace the bushings and pins. The good news is that the bushings and pins can be replaced without opening the transmission
So, $70 worth of parts replaced all 5 bushings and pins. I opted to replace all of them since I plan to keep the car for a while. The simple fix for just the lean can be done with just $25 worth of the bushings.
Tools and advice available for anyone with this problem. the sucky part is that it requires removal of the transmission.