This sounds similar to what I went through with my 250SL. It turned out to be a blocked return line (although it could be other problems). My car would run great on the highway, or as long as I kept going at a decent RPM. However, after 15-20 minutes, the car would stall upon slowing down to a halt. Someone placed a small steel ball in my return line, right near the injector pump. I had replaced all of the hoses, along with the fuel pump. Further I drained the tank twice. If I had listened to others advice on this site and followed through by checking ALL FUEL LINES, I would have discovered the ball much earlier (why someone place a steel ball in the line I will never know).
Hopefully, your mechanic will discover the culprit early!
Best Regards,
J. P. Mose
1968 250SL
1987 560SL