Ideas more than actual diagnostic advice, but..

Had a similar problem with our '91 300E. It would crank for a couple seconds, then catch, run rough for a few more seconds, then slowly smooth out as the rpms came up. Digging through the records, it looks like they replaced the fuel injectors as well as the O2 sensor, and now it starts fine. Might want to check the O2 sensor first, since the injectors can be fairly pricey.

Could also have been the mixture, I believe they had to reset the lambda when they did the injectors, and that may have helped as well.