Unusual Intermittent Steering Wheel Vibration
 

Hello folks,
My first time posting on here. I got a problem that is just stumping me. I have researched it, even had a mechanic check the front end... can't seem to find the problem yet.
I'll try to explain the best I can:
Model: 1993 Honda Civic Stock.
The Problem: Usually at highway speeds the steering wheel starts vibrating violently; to the point you have to slow down and pull over. Steering becomes sketchy and it feels very unsafe.
However, this problem is only intermittent. It seems to happen from time to time when driving on a highway for 30 minutes or more. It doesn't always happen.
And when it does happen, only when I'm cruising with foot off the pedal.
No matter how violent the shaking, if I apply my foot to the gas, it smooths out.
Any thoughts? Any feedback would be much appreciated.