Hm, Gigantoad really seems to be right. I overclocked my gpu and reached the 75 cap at all times in my two test projects. The stuttering is gone. But that would mean that even slightly dropping below 75 frames makes the game almost unplayable. Or is somehow related to the rift being set to 75 herz?
Anyway… my problem seems to be solved - somewhat. The fact that we are talking about an almost empty scene here is kind of frightening :-/