Sounds like you are having the problem that I'm now having with a 6800gt. Different drivers do not help, but going back to my 5900 Ultra did.I'm thinking either my power supply is overwhelmed by the 6800gt, or there's some new bug in Horizons. I have not tried re-formatting, since every other game plays fine.
I have a second pc that has had no problems at all with Horizons, it currently has a 5900 Ultra init also. I'll try sticking the 6800gt in there today.
Guaran