Three times in the last six months while travelling at about 70mph on the autoroute on a slight curve a vibration set in which gradually got worse until the car was undrivable. Slowing down did not stop it. The only way was to stop and then restart and the problem went away for a couple of months and then returned in similar circumstances.

Most of the front suspension has been replaced recently and the wheels balanced etc.

I can't understand why it happens only infrequently but it is disturbing

Any ideas?

Have you checked for free play in the CV joints and the inner tripod bearings? Grab hold of the half shafts at either end and give them a good pull, or carefully use a pry bar near the ends of the half shafts, levering against a solid bit of subframe, upright or similar. It can happen that the halfshafts sit centred in the bearings most of the time, held in their well-worn 'grooves', then a certain combination of suspension load/movement knocks the shaft out of these tracks, at which point it rotates eccentric and so vibrates. Because it's then being 'flung' out of its usual orbit, it won't drop back in until you're going sufficiently slowly for the shaft to drop back into centre.