(Sorry I'd written this out yesterday and forgot to post it lol)
Uncertain what you've tried and what you haven't, but have you checked your active clock speed of your GPU whilst running the game/benchmark?
Use a conjunction of either/or/both GPU-Z on the Sensors tab, and MSI Afterburner, see what your GPU is running at.
An old fix for high CPU usage which would often cause stuttering with high CPU usage, was to alt-tab out whilst running GTAV and start Task Manager, look for GTAVLauncher.exe and close that process, tab back into GTAV and it may fix it (I actually had the issue arise recently, even though it's an old problem).
Another thing to try would be to go into nVidia Control Panel> Manage 3D Settings, click on the Program Settings tab, and look for GTAV.exe (or add it if not on the list), look for Power Management Mode, set it to Prefer Maximum Performance, and apply.
Give that a shot and see how it goes.
What PSU have you got, out of curiosity? And is GTAV the only game that does it?
If we knew the active frequency of both your CPU/GPU, we'd be able to better try and troubleshoot, the CPU is in no way a bottleneck for that GPU, it could be a simple driver issue, it could be something else.
Download the latest drivers straight from nVidia and run a custom installation, check the Perform Clean Installation and install whatever you use. (if you did the nVidia CP stuff prior, you'll have to check that hasn't changed with the Driver Update)
If the slowdowns are at random and happen for a long period of time, the hardware is throttling for some reason, or the hardware simply thinks it isn't needed to be that powerful, so is ramping down (as temps aren't the issue) or simply isn't being put to use correctly (drivers).
Also I'm assuming that all the external power leads (PCI-E and CPU) are plugged correctly into the Mobo.
The reason HalfOfAKebab asked about graphical settings screenshots, I think was just to mull over your Advanced and general graphics settings, resolution you've been adjusting to/from etc, as resolution makes a difference on CPU/GPU usage.
wow that is weird, i get similar FPS on much lower hardware at minimum requirtements mostly (gpu is midrange).
CPU is only a pentium, now im thinking if it's even worth upgrading if higher hardware don't change anything (250€ would be useless to waste), only real reason to upgrade would be going over to windows 10!
Honestly it depends on which Pentium you have, the newer ones have four threads, so act more like an i3, but not quite (lacking instruction sets, cache), and more threads really help in games like GTAV (stuttering would be easily shown on GTAV with a 4.5Ghz+ G3258 for example), however GTAV really favours quad cores, as Hyperthreading isn't near as powerful as a full core (think of the extra threads as 25-30% as powerful as an actual core).
With a Pentium or i3 (depending on how old), you're also more likely to have legitimate CPU bottlenecks the higher performance Graphics cards you run, rather than simpy slightly less FPS thanks to generational IPC improvements (the card shown in the chart linked isn't as powerful as todays, as have their been marginal improvements in CPU performance).
Also which resolution you play also makes a large difference on your CPU requirements.
All you'd have to do is Google/YouTube your current model vs the model you'd planned to upgrade, to see the differences in performance (or similar models).
Hope this helps any.
[EDIT: (lol took me over an hour to add stuff to my post)]
btw when it comes to increasing fps in gta games then its like black magic to get it done, no certain way to do it!
If you don't want to waste nerves you better get used to random fps drops in gta games, they were made for consoles mostly, it's so annoying if people stare at fps and whine if it drops under 60, 30 fps is already playable!
I dunno dude, you really do notice drops to 30-40 from 60+, I have all my visuals on max including Grass (love that Ultra Grass man) and whenever I go near the hills my FPS takes a massive hit (naturally) to the 30's/40's I can really feel it, makes me cringe.
Everyone is diffferent, and we as humans adapt to what we're used to afterall. I used to be able to smash out the Xbox360 all day, PS, N64, Snes/Mega Drive, Atari, ZX Spectrum. Now I play older games on older hardware it gives me massive headaches as I've become accustomed to the framerates, smoothness of pixels of my current setup.
I can't even play the PS4/Xbone now without feeling the difference.
You'll always get drops of some extent, be they due to other player interaction, large scale Physics processing, but drops that happen for long periods or severe short-term drops like stutters, are largely hardware/driver based issues and you shouldn't be content playing a game that stutters/drops that consistently/badly (moreso on hardware that should be more than capable) unless it's down to the fact that you simply can't afford anything better, lord knows I've been there.