Sounds like a bug in the game to me. I have seen similar things in quite a few games. GTA4 was designed for consoles and the port to PC was not really great. Make sure you have all the patches.
that's what I thought
Just for peace of mind though - if I were, say, experiencing a failing GPU, I would be seeing many issues, correct? And since a stretched line is generally a sign of vram issues, ti would be rendered over everything instead of uder everything right?
And shadows wouldn't affect it?