I just don't understand why you think that if it works for you then it must be a-okay. There are thousands of people having the OP's problem, which points to an issue with the game itself.
According to official sources, this game sold over 300,000 units in the first month and over 2 million as of Dec 09 according to take-two. Even if 10,000 people were having critical problems with the game (an unrealistically high number), your only looking at 0.5 percent. But lets assume those numbers are fudged horribly and maybe they only sold half that, the amount of people with critical problems (crashes, driver issues, etc) would still amount to only 1 percent. I would like to say that more than 10,000 had problems, but there are absolutely no sources, official or unofficial, that can qualify those numbers. Hell, even if your scoured the forums and counted every post that had a problem, you could maybe quantify 500-1000 or maybe 2000 if your really good and spend the whole weekend searching, but any more than that is unrealistic.
I think the game prob has some issues, all games do, but I am hard pressed to believe that the game has problems of this magntitude. It just doesn't fit the data. The game in general has been well recieved and for the vast majority of people runs well (which includes the Steam download). I feel bad for the OP but theres likely something else going.