I posted this on reddit as it's the ONLY fix that has worked for me so far...
I've tried everything over the last 2 days to fix my frame stutters throughout the city (GTX 970, i5 4590), when it was pointed out to me in another forum that being 'stuck' at 59Hz in game (when I'm using a 60Hz display) could be the issue.
The fix is this:
*Google Custom Resolution Utility 1.2 and install it. I got it from here:
http://www.monitortests.com/forum/Thread-Custom-Resolution-Utility-CRU?page=1
Follow the instruction on that page, but in essence you just make sure it says 1080p/60Hz under "Detailed resolutions", then if your an Nvidia user make sure you select "Custom extension block" for custom resolutions to work properly, in the Extension block setting.
Click OK, then run restart64. The screen will flicker for a second so the change can take effect. You don't need to, but then you can reboot if you want.
In GTA V, under "Graphics" you will now see that 60Hz is enabled - and I guarantee the game will be smoother.
The ONLY issue I have found is that in my case (connected to my Sony 60" TV) that I can't output sound through my TV - not a biggy as I use my headset 90% of the time - but worth mentioning none-the-less.
If there are any issues, you can always run 'reset all' and everything will go back to the way it was.