What is causing this Vibration? Please help.

[Meza]

The car is a 1993 190E 2.6L. I feel vibration between 60 and 80 miles/hr on the steering wheel. I did the followings:

1) I went through 2 sets of 4 new tires.
2) Did alignment.
3) Balanced and rotated tires several times.
4) Checked worpage on rims. (Was fine)
5) Completely checked suspension and steering components. They were fine according to the person who did the alignment. He is a pro-suspension/alignment specialist with over 40 years experience in that fiels.

I accidently found out that: While driving through the mountains on a long trip, I noticed the vibration is greatly reduced while steering left at 70 miles. Any ideas how could this happen?

I suspect my drive shaft or center support bearing, but I don't want to replace it until I make sure it is not the problem.

I am really impressed with this board and how organized it is. Thank you all in advance.