Check the front of the rails and see if the bumps where the front clip slides into are flatttened, for if the front clip on the body is bent up upwards.
put the seat all the way back and pound he top of the clip through the rail with a drift and hammer to bend it downwards and thightening up the seat.
Vee ave vays of dealing vid your kind...........
Any of this fix your problem?
http://bellsouthpwp.net/m/i/miales/rockingseatfix.htm
Cosmos Black/Black 1995 540iA M-Sport
BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build