Vibration - Ideas?

Putting this out there for ideas basically.
I have an intermitent vibration when I get to about 70 - 80. (on a private road of course)

FEELS like wheel balancing.. but had them done recently ( I guess I could have bashed a pothole but don't think thats the cause)
Just replaced both front discs - which got rid of the braking vibration and I THOUGHT would get rid of the driving vibe - but on the motorway, its still there.
What could be causing it outside of Wheel Balance and new discs? Seems to be worse under load, i.e. accelerating up a long motorway drag, as opposed to coasting where it doesn't seem bad at all. Engine revs freely.. no weird vibes or rattles from that.. am stumped!?