Turn off SLI & make sure the refresh rate in game matches your monitor, then enable V-Sync, it does wonders in this game. I know in other games its crappy, but for GTA 5 seems to work very well, it fixed my stuttering. Also, I know you have a powerful GPU, but turn any settings in game that are on "Very High" down to "high", make sure you have 8x anistropic or something I think it is....most other values like 4x 16x seem to cause issues. If you have mods installed that affect AI behaviour like traffic, try taking them out as a test, some are not optimised well enough yet.