Drawdown frame rate (Grand Theft Auto 5)

The forum there is a small performance issue,tell.
Config PC:
Intel Core i5 3.0 GHz 7400
8 GB of RAM
Geforce GTX 1060 6 GB VRAM
Windows 10 x64 Pro
The pirated copy of the game.
Graphics settings are all max except:
Antialiasing Msaa: off
Reflection Msaa:off
Shadow quality: high
Soft shadows:Max. gently
The effect of depth of field:off
DOP. image settings all turned off.
Full HD 1920x1080 resolution.
Kolichestvo FPS on my PC with Vsync enabled stable 60 frames.Without Vsync, the FPS will scale 100.'ve done system monitoring using Msi Afterburner. CPU, graphics card straining not much, at all stages in different RAM and loaded about 6 GB. The increased temperature of iron is good. Freezes, no lag detected....
And now about the problem itself.Play somewhere 1.5-2 hours. Notice for example the average drop FPS, for example from 60 to 40 or 30 fps, the human eye can see. Time posadki somewhere 0.5-1 seconds. Ie everything happens at lightning speed, but to see is not difficult. Do system monitoring during subsidence, the iron is again not naparitsja, overheating is missing. Such subsidence does not always occur, with different periodicity, in any gaming areas, and after about 1.5 - 2 hours of the game in different ways. You can certainly not pay attention, but how strange...
Yet something figured out. During the fall of fps, quickly open the task Manager.
Hard drive 100% loaded.And ship it service '' the System and compressed memory'', Oh but it's Windows 10). Explain whether load of the disk to affect the drawdown of FPS during the game? Hmm, the other games didn't noticed such problem.
Or the problem lies in anything else?
Share your thoughts.