Ugh. Bell ringing here. I had (and still do i think, haven't played gta3s for a long while) the same problem with the disappearing road/land textures. and i got it in both liberty city and vice city gtas.
i might look into it this night. i think there was a patch for at least vice city, which was supposed to solve the problem, but it didn't.
i think i'll try changing the drivers, messing around with game's graphic settings (trails, framerate limiter, view distance...), vid card's antialiasing and anisotropic filtering settings.