Intermittant vibration

I was unsure whether to post this in the transmission section or the suspension section as I'm perplexed as to where the problem is coming from. 95% of the time, the car (2.8 FL) drives perfectly well, then for some reason the car starts to vibrate. After slowing to around 55mph and driving a further 2-3 miles, the vibration dissapears and might not reoccur for weeks. I "think" I can get it to start the vibration by acceleration hard in "S" mode from a standing/slow start, but this isn't 100% repeatable.
Anyone got any ideas what the cause could be?

To add some more info, the problem only ever occurs (or at least becomes noticeable) above around 60mph. I've tried coming to a complete stand on the hard shoulder before setting off again and the problem still persisted, only to disappear 2-3 miles later. Also tried moving into neutral and allowing engine revs to drop/rev engine higher but vibration persists at same resonance. Help!

[RanjS8]

If its intermittent then its more than likely drivetrain related especially after hard acceleration....I would check get the car up on axles stands and do the following....

1...check the inner cv joints, and out cv joints for any clicking sound....
2...check the wheel bearings for any excessive movement...
3....also might be worth checking the output flanges for excessive play or leaking oil....

4....if that seems fine....then i would check wheel alignment and wheel balance before going the the suspension route as that tends to cause knocking noises....

MOT is due this week so hopefully it might shed some light on the problem; I'll get the CV joints & bearings thoroughly checked while on the ramps. Hoping it might be wheel balancing - cheaper than a new set of CV joints!