Advice on C280 probs

I'm new to this forum - it's fantastic. I've used the search function to find answers to my problem, but I still can't nail anything down. So here goes.

I have a '99 C280 (high miles around 95k) which has a transient shaking underneath the floorboard, sometimes the gas pedal, and sometimes the steering column. At times, all three together. It usually starts when I hit about 60mph on up, and stays as I decelerate below 40 mph. The trigger speed is about 60. Where I live, there is only an independent shop around which has been awesome. My mechanic has been baffled. I have new tires, high-speed balance, new control arm bushings, tie rod ends, etc. over the past year or so. Hasn't helped. He thinks it is something in the power steering system, and can't find anything wrong in the suspension or driveline.

Can a bad steering pump or gunked up steering fluid cause these symptoms, or maybe steering knuckle (as I saw in a previous post)? Maybe a transmission problem as opposed to the actual driveline?

Trying to get it fixed here in semi-rural New Mexico without having to drive 200 miles to a dealership.


TIA

Gary Batlin
Pie Town, NM

I should have also added, there is a lot of slop in my steering wheel - sometimes it feels firm while at other times it seems to get loose a bit and I seem to over-correct driving down the highway. Is my suspicion correct in that these things are interrelated? The mechanic adjusted the steering box and it seemed to tighten it a bit, but not much.

Gary B.