What do you mean by "slips into gear"? Do you mean this?: After stopping and then pressing on gas, the rpms go up but the car doesn't move and all of a sudden it jumps, just like dropping the clutch on a manual at higher rpm. First thing you should do is, check if there is a dip stick for the tranny underneath the hood, if not, it's sealed and you'll have to keep on reading my long and boring post. You can read more about it here:
http://www.pelicanparts.com/BMW/techarticles/E36-Tranny_Fluid_Auto/E36-Tranny_Fluid_Auto.htm . If that's what happens, it's the oil. I had this problem and wanted to get it fixed at a dealer, since the tranny is sealed but the closest appointment was a few months away, so I called a few tranny places but they wanted to do a full diagnostic and get a lot of oney from me. Finally this shop agreed to do it and the problem went away after 10 bucks and 10 minutes of waiting. Anyway, you can do it yourself only if you can get the car up in the air and leveled. I tryed but because I used the wrong wrench, I kinda f* up the bolt, plus it was really hard to get to if you are on youre back underneath the car. After buying most of the stuff needed, I decided that it wasn't worth it and found that tranny shop. I would recommend you do the same since 10 bucks aren't that much, you will spend more on the oil, the pump, the stands (if you don't have them) and last but not least, the wrench. Also, the guys in the tranny shop know what they are doing. Almost forgot, don't go looking for regular mechanic, youre better off just calling the closest tranny shop that can be found in the yellow pages and driving to the one that agrees to just add the oil and not the all day long inspection. The tranny pan should be between 25 and 50 degrees C, too hot and they'll add more than needed, to cold and they'll add more than needed. Anyhow, I wish it is the problem I've described and not something worse. Sorry for such a long post and I hope this helps you out.