Well, it's pretty self-evident that a degree of intelligence far beyond the norm will isolate you socially. I guess the distinction between dysfunction and social awkwardness/remoteness then comes into play. I'm not sure it would be a major issue in a society which didn't aim to replace all meaningful human relations with trivial socialization. At least, such high IQ types would be regarded as strange, but encouraged to use their IQ towards greatness (the great composers, philosophers, etc spring to mind). If they were in the "extremely high" range, you would hope your eugenics was good enough to produce enough similar individuals to prevent total isolation. However, since it isn't an exact science, uber-intelligent "freak" types (150+) could always occur, and they would most likely be completely isolated, "unstable" of you will. So be it.
I'm not sure why you think it's possible that different ethnic groups would be able to build/maintain a lasting civilization. I'm not sure that a ruling class of high IQ individuals would be able to prevent the type of problems we know are inherent to multiculturalism. The absence of democracy in this scenario would simply remove political power from the lower strata; however, revolutions and the like are still possible without it. Of course, if everyone is on the same ideological page, no worries. We know that this is never the case, and ideology varies between class, caste, ethnic group, religious affiliation etc, quite fundamentally.
I'm struggling to find a plausible scenario where this would work.