Unplayable shuttering after some time

Hi
I really enjoy this game and it runs really nice for me, smooth 40-60 fps on high settings 900p>1080p res. but just for a first 30 minutes into gameplay. After that time (sometimes its more sometimes less) game will start shutter and massive fps drops, texture bugs occur.
I know that my PC is nothing special, but it runs nice and smooth and problems appear only after some gameplay time. Is it something RAM or hdd related?Does somebody know what my problem is and what can i do about it?
Thanks

i5 4460 3.40GHz
8GB RAM ddr3
MSI R9 380 2GD5T OC
1TB HDD

I hope you will try CPUCores :: Maximize Your FPS
Its a software and it worked for me

8gbs ram are enough, just be sure you don’t have too many apps running in the background, and also be sure to have free space on the hdd for memory swap.

When I play I always shut down everything on background and currently I have 90GBs of free space on HDD.