My vote goes to the steering damper
Duke is right.
I experienced the same thing a couple of days ago.
I had just entered the highway and at 70km/h, I hit a small bump and there was a slight instantaneous vibration felt at the steering wheel.
Accelerating to 100km/h, I hit another small bump but this time, the steering vibration was much more pronounced and the vibrations lasted about 3 seconds.
I pulled to the side and did a quick visual inspection.Nothing was amiss.
I am going under the car to check the steering damper (that's my guess coz it doesn't seem to be absorbing the forces transferred to the steering).
On my car, the damper is connected by 2 bolts.I believe, a good way to test it is to remove the bolt from the drag link end (either side is ok i guess) of the damper and test the damper for resistance. It should be like a miniature shock absorber. If there is no resistance in the damper, it's shot.
It's a cheap part for my car (like 20-30 bucks) and it looks like a quick DIYer.
I'll find out soon enough.
Hope it helps.
Babybenz 1991 190e 1.8