Shifting can be affected by some of the vacuum hoses in the engine compartment. On my 91 300D 2.5 there was 1 on the front (on the engine above the fan area) that I had trouble with. So don't think it must be on/near the Transmission. Also, I have a 722 shop manual on PDF that is free if you want it. Send me a Private message and I'll send you a copy. (same offer to others).
Good luck with your problems, it's a great car !