i think the OP is talking about small stutters of low FPS. i get them too in certain areas of certain maps, but usually only the first time i drive it. maybe there are files being loaded from the hard drive or something and it momentarily slows down.
my definition of "runs fine" is 25+ (if you look at my gpu, you'll see why (whats even sadder, is that it's still the most powerful gpu i've had in any pc)
Looks like amd cards get better fps than nvidia cards because I have a core i7 3770k and a evga gtx 670 FTWsli but using only one card my fps drops after the bridge to 40. It would be nice to get a sli profile and use both of the graphicscards.
I tried it but it didn't give any performance increase except on empty grid map. On other maps beamng was using only 50% off my graphics cards.