the BGN pack first came out. If I recall correctly, the solution is to
do a clean install of N2, then the track pack, then put all your custom
cars and tracks back in the new installation. I know it's a pain, but I
think that was the solution.
By the way, you might try running nasrend -dma -4--this definitely makes
a difference in tracks like Texas and the new short tracks. Coming out
of turn 4 in Texas, for example, without using the -4 switch gets choppy
on even the fastest machines for some reason.
Good luck,
ANash