I am new to the BMW community, I just happened into a 1989 535i. It's not exactly mint condition however it has great potential. I work on performance domestic cars for a hobby( my other car is a 5.0 Mustang) but all this BMW stuff is new to me. It has an automatic transmission, and at times it will just shift out of the final drive gear then right back into it. This usually happens around 45-65 mph. It does it intermitently and sometimes not at all for a few days. I really am trying to avoid having to take it to the shop if it is something I can take care of on my own. Also any recomendations as where to get performance parts, and tips on tuning?