This video is an example of what's happening to me

http://www.youtube.com/watch?v=fhdB3wJ3hgc

Basically, in large battles, a large portion of the enemies will not render, so I will have to rely on the compass to kill them. Additionally some of the enemies are also un-hittable, causing allied npcs to swing at them endlessly.

Never "seen" an invisible enemy, but I have run into the occasional issue where I can't manage to finish off some enemy - they suddenly became un-killable or something - most times they just stand there doing nothing

I did have one (and only) occasion with an invisible falmer, but that was back when the game was on version 1.1 or maybe 1.2, in Nchuand-Zel (the ruins inside Understone Keep). No other incidents eversince.

Maybe the time has come for a game cache verification -that's my best idea for now.