If you are checking your fluid level with cold fluid, the level should be about 10mm below the add fluid mark on the stick. Checking hot fluid requires driving 20 to 30 minutes to bring it up to temperature.

I agree about changing the fluid in Torque Converter and Transmission every 30K. It is a trivial DIY exercise. Transmission in my '92 300 E shifts the same at 245K as it did at 100K.

Finally, your problem could be as simple as having the transmission shifter bushings replaced. Cheap nylon bushings, but a bear to install. Pay to have it done. Mine were toast somewhere south of 200K miles.

Steve