I dont think its hardware, as my 8800GT has no similar problems with any other game, including ones that are more harsh to it.
Since i switched to windows 7, i havn`t had this problem, so can only assume its driver related.
First thing that i would do, is get driver cleaner and a new set of drivers for your card (roll back a few versions).
Start in safe mode and remove your (asuming nvidia?) drivers, then start normally and try your new ones.
Hopefully it is a problem wich is easy to solve, because i spend some 7 months with working at this track. Had no problems and suddenly it behaves like this. So sad i have no back up for this one. Yesterday i was so fed up i almost deleted the whole track. Luckily i didn't.
First i have to know how i can get a higher frame rate. I saw some tracks, like the Nurburgring and some other tracks wich also have a lot of trees around, wich have not such a huge impact on the frame rate.
It could be that your graphics card is failing, but it could also be a problem with the power supply unit. Last month my computer would halt and all signals to the monitor would cease whenever I was running any 3d graphics program (including BTB). My card was fairly new so I replaced the PSU, and now stability seems to have returned.
Of course there are many variables so it can be very hard to narrow things down, but other hardware faults are worth considering.