I don't know what frame rate capper you guys are using, but it's essential with GTA IV.
I used to use dxtory. It served me well for a number of years. I'd heard about rivatuner, but it took me a long time to get round to trying it. It's not as easy to setup as dxtory, but it's FAR more precise and well timed. It's VERY good for GTAIV. Nvidia driver level capping has been broken for a while now..
I know people want to run at high frame rates, but the problem is the game is very difficult to run at 60 fps and most have a 60hz monitor. And you need to cap at something you NEVER drop under for the game not to stutter. I myself use a 120hz monitor and use 1/3 refresh vsync forced from nvidia inspector with 40 fps cap from rivatuner. It's very important that you cap as well, even though the vsync caps at the same frame rate, the game will stutter otherwise.
If you want to see how truely fluid rivatuner and multiple refresh vsync can make the game, I suggest capping at 30 fps with rivatuner and using 60hz half refresh (not adaptive) vsync forced from nvidia inspector.http://i.imgur.com/nsUR2KX.png
Another thing that is important, is that you alt tab out of the game disable the HT cores from an i7 in task manager. That also seems to make the game stutter. I think this may be nvidia driver specific though, and it sometimes does it even with HT disabled from the bios. That is to say, even with an i5, you have to run on 3 cores instead of 4 for some reason. It doesn't matter, the game only runs 3 threads anyway, so you don't loose performance. You can even alt tab back again and enable on all cores again. I know it sounds wierd, but for some reason you just have to mince around with affinity settings once it starts running for the game not to stutter sometimes.