GPU usage drops while playing GTA V

[RaphaelSandu]

So, I'm having frame drops while playing GTA V in most of the map. I've decided to check the temperatures and usage of the CPU/GPU, and turns out that sometimes, GPU usage goes from 70% to 40% and even 0% in some cases (entering stores, ammunation, etc). In some parts of the map, the GPU and CPU get between 85 and 96% of usage, but the temps are fine, both CPU and GPU doesn't get over 75 Celsius while at peak. The graphic settings are the ones recommended by Geforce Experience.

Could his be a bottleneck issue? I've also installed the game on my 500GB hd, could this be what's causing the problem?      

 

Sorry for any grammar mistakes, I'm not a native speaker. 

PC Specs: I5-4590 | B85M-D3PH | GTX 1060 6GB | 8GB RAM 1600Mhz | SSD 120GB | HD 500GB | EVGA 430W