Hi! I have an 83 300D that shifts irratically between 1st and second. When it upshifts into second, it sometimes gets too much power and jerks forward, other times it's fine. I think it has something to do with the turbo kicking in but i'm not sure. What the bigger problem is, is that today it started clunking VERY loudly and shaking the whole car when it downshifts (while DECELERATING) from second to first. Again, it only did this some of the time, and I should also note that the clunking felt like it was coming from the BACK of the car, not near the transmission. This totally baffles me. I've gathered from other posts that it might be due to problems with vaccum, but that was a bit too over my head for now. Any thoughts/advice?

 

The clunking might very well be a worn flex disc. There are two of these rubber discs; one between the transmission and the driveshaft, and the other between the driveshaft and the differential. Go under the car and inspect the discs. If they're rotting away, replace them very soon.

As for the the harsh upshift, usually slightly releasing the accelerator immediately before the shift helps smooth the gear change. Also, if the trans fluid and filter have not been replaced recently, get that done soon. Replace the fluid pan gasket as well.