Ok, just to follow up on the driver issue...
I downgraded from version 182.50 (latest) to 181.22 (january 2009)...
Next I stress tested the game for over an hour (meaning I played nonstop for that length of time
So far, NO STUTTERS... AT ALL
Performance held stable during the whole session... frame rate was high, and not even the rain, which usually lags my system, made a dent
Worked for me, might work for others... let me know if it works for you
Here are specs, if you're interested:
GTA IV 1.0.3
Win XP 64 bit
Geforce 8800GTX 768MB, running @ 3072x768 in-game resolution (triple screen)
Intel Core i7 920 CPU
6GB DDR3 1600mhz RAM
Intel DX58SO Mobo
Because of the high resolution, I usually leave my graphic settings at medium-ish... textures, water and reflections are medium, shadows are high, sliders set at around 25%. definition OFF, Vsync ON
Running this rig at a more normal resolution might give enough headroom to push the sliders much farther forward, but playing on 3 screens is well worth it