driver side electric seat moves about 1 or 2 inches when I accelerate or brake

Unfortunately this is a common and annoying problem on the up to 2000 model Z3s. There are improved parts out to repair this condition. Most BMW specialty shops are familiar with this problem. Here is a SIB (TSB) from BMW regarding this problem: Interior - Seats Have Slight Front/Back Movement SI B 52 02 03 Seats November 2003 Service Engineering SUBJECT Replacement of Seat Adjustment Isolator Pad MODEL Z3 SITUATION Some customers may complain that either seat seems to slide back and forth slightly under braking and acceleration. The urethane isolator pad used in the fore and aft seat adjustment assembly may compress or wear over time, causing excessive play in the mechanism. CORRECTION The material used in the isolator pad was improved in production in January 2000. A parts kit and repair procedure has also been developed to eliminate the need to replace the seat track assembly on cars that exhibit this problem. Zee

This is a common problem. There is a bushing on the seat that gets worn out and creates this problem. They sell a kit on ebay for about $29.00. Now to install it is a diferent story. Mine has the same exact problem. Already research the problem.

had the same problem with my car's seats (both passenger and driver). Brought it to a BMW dealer in 2005 and they replaced the seat tracks. Would have been expensive but they got BMW to cover the entire cost.