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!?

[Conan_the_Librarian]

Any vibration when revving the engine in neutral?
If not then it's probably a rotating mass issue, so with new disks and pads and recently balanced wheels, that leaves Wheel Bearings, CV joints and half shafts. I gonna plumb for CV joints wearing out. Have you ever had to replace split CV boots?

Definately something rotating. Engine revs freely and smoothly.. have tried revving engine when stationery - lovely.. and also putting the car in Nuetral when coasting and freely revving - that also makes no change. I feel the vibe still there, but not as bad as when the car is pulling.
Might plumb for a quick rebalance incase I have messed something up.. but then look into CV joints.
Haven't replaced any split boots, and all looked well when changing the discs last weekend..

small update on this. Had another look around the wheel wells to see what I could see.. nothing obvious. No loose joints, doesn't seem any movement on the wheel. CV joints don't click to they too seem ok.
Went to the garage and thought I would inflate the tyres a little firmer than normal - and that seems to have made an improvement to be honest. Now not as pronounced and smoother at the aformentioned speeds. So that leads me to balancing then I guess.. (why else would the vibration reduce by upping the typre pressure!?)

one question - I have a set of standard 18" sprt wheels with 245 tyres on them. what is the best tyre pressure. At the moment I have them in my opinion quite hard up around 36psi, which seems to have reduced the vibe (although clearly I need to go get them balanced and thats not a fix!)

[PsYcHe]

I'm running mine at 40psi as I do a lot of motorway runs and that seems to keep them happy. I found I got vibration if they dropped to around the mid-30's. Nothing serious, just enought that I noticed.

Might vary from tyre to tyre, but seems to be the same for my original sport wheels with Dunlops and the replcias with Runways (cheap, but effective).

oh.. well I am probably a bit soft then (!) - I'll run 40 and see what its like. I always sit on the M'way too, with about 5 miles each end to Home/Work.
Defo seems a little happier being at higher pressures, just wasn't sure what the max / recommended is.