I have a similar problem, so probably the fixes are the same as above. My '81 300D non-turbo has a rough acceleration. It idles smooth but the first part of acceleration (from 500 to 1000 RPM -just a guess b/c no tach) is rough. After about 1000 RPM it smooths right out. I thought it was my drive shaft because I know that needs balancing, but it is noticeable also while idling in park or neutral. I know a few of the valves are bit tight as I checked them the other day but did not adjust them (my mechanic is much quicker and equipped for that job). So, I'm thinking motor mounts or valves. Anything else worth checking?