That used to happen to me too...Untill I uninstalled the game xD
But check your Draw Distance, and your graphics settings.
I remember your problem started to happen to me when I installed a highway for VC (It was huge, covered whole VC).
I think this have been answered several times before
But well, check that gta3.img is un-ticked as "Read Only", and either delete, or rename the file you want to replace.