I have experienced this twice with my '82 240D and '84 300SD. I replaced the worn rotors in front with "cheaper" ones...ATE and I thought all was well. A few thousand miles later, I get pedal vibration when braking. I took them to be resurfaced...but the within a month the problem came back. I decided to replace them with BALO rotors. They have their own steel mill and control the quality of the steel as they see fit. I'm really impressed by them. They have been on for several years now and no problems. Same story with the rear rotors...put ATEs and got warpage...put BALOs and problem cured.

On a side note, it could be that you are getting uneven braking causing one rotor to get overworked and overheated causing it to warp. If your mechanic did everything properly and all the "mechanics" of the brakes are working as they should, it may be that the rotors put on there are just of poor quality.

Nolan