I had the same problem on my 87 300E. After a lot of troubleshooting, I finally found the problem. It was the E.H.A. valve, and I beliebe that's a fancy name for a fuel pressure regulator. It's a Bosch part and a bit pricey for such a small thing($200-250) (Dealer may charge $450), but very easy to replace. The "bog" on mine occurred only when the engine was warm, and only when starting out in second gear. (Shifting to "2" made it start in 1st, and it didn't bog then.)