Ford Galaxy vibration ?

I have a 2005 1.9 TDI galaxy and on a recent journey to go on holiday, after about 3 hrs of motorway driving suffered from bad vibration which I can only descirbe as feeling like a bad miss-fire.
After stopping to check wheel nuts, reached destination ok and was ok for rest of week.
On the return journey the same thing happened again only after several hours of driving.
There does seem to be some minor vibration just under 2000 rpm but the major vibration has not re-occured. Looking at similar problems it points to the Dual Mass Fiywheel, but why should it become worse and then appear to get better again