Well, in my case it's a bit of both. The engine itself is a bit unstable at 3000 rpm, but barely noticable. As soon as i let the clutch in, the driveshaft just amplifies the vibrations. And i'm pretty sure the driveshaft and clutch are ok, as they have been replaced by a pro porsche mech and really cleared up most of the really annoying vibrations and sounds.
By the way: i agree on your point that in the late 70's it wasn't that easy to filter out any unwanted vibrations, but i hearded that the 944 and 951 have both been tested on stuff like vibrations and that they made great advances there compared to the old 924. I can tell you, if my 951 vibration where stock, i would NEVER want to drive 924
But they weren't...