My first guess (never having owned a 190sl) is the various bushings on the shift linkages. They're cheap to buy, a bit of a pain to install, but make all the difference in the world. Also check if the tranny needs gear oil or ATF and make sure the right on is in the box.
However, you'll find lots more 190sl owners on the veterans list (www.mercedes-veterans.com). This group of folk is good, but for the veteran cars, it's hard to beat an on-line group that has people that have owned and/or maintained them since new.