You didn't say if this is only when the car is cold or not. I will assume that it happens all of the time.
First off, how old is the ATF? Since the car is new to you, I would drain the trans AND torque converter, install new AT filter and fresh ATF. Personally I use Mobil 1 Synthetic.
You may also want to review this older thread:
500E Transmission Woes?
Look at what Michael has to say about adjusting the bowden cable. Now this may be the linkage you were adjusting, but make sure it was set correctly before you started. Remember, one "turn" will make a big difference.
I played around with the bowden adjustment along with the vacuum modulator. Under "normal" driving, the transmission shifts exactly where I expect it. The shifting is so smooth, you can barely feel the first to second and third to fourth shift. The second to third shift is a bit firm, but no matter what I do, I cant seem to soften it up.
Hope this helps.