Until I had a new gearbox (a new 75/Milano gearbox-diff) put in my '84 GTV6, I'd never noticed or ever thought about any vibration problems. For good measure, I had the clutch replaced too. But not the flywheel, apparently. (The old gearbox had the usual syncro problems, plus I wanted a better 1st gear ratio). All was fine for a week, then a terrifically loud banging came from under the car, vicinity of handbrake, speeding up with revs.
I took it back, and the guy said he couldn't figure it out, but replaced a 1-1½" circular bush-type thing where the driveshaft joins the clutch assembly. (Sorry, he showed it to me and told me what it was called, but I've forgotten its name; he replaced it with another used one.) This stopped the banging, but instead it had an undriveable degree of vibration - above 2500 rpm it was frightening - just sitting stationary the whole car shook. He took the car back for several weeks, and then handed it back pretty much unfixed. I took it somewhere else and had the entire drivetrain balanced, which improved the vibration about 80%, but left it still unpleasant to drive - the vibration is now mostly on hard acceleration, with the gearstick shaking in your hand.
It's now gone back to the original guy, who says he feels bad about the whole thing, and wants to keep it for several weeks to "really sort it out". He says there are so many different sources of vibration in a GTV6 driveline it's hard to know where to start. I haven't heard from him for 4 weeks now, so I'm not too hopeful. In the meantime, I've bought another GTV6, partly so I'll be able to have some happy GTV6 driving experience before I die. Any thoughts, not dwelling unduly on the fact that I'm a dill to trust these guys, would be welcome. However, reading other posts on this subject, it seems depressingly clear that every time you replace bits in the drivetrain, you run a very big risk of unbalancing things.
DaveR