Performance Issues Radeon RX-560 + AMD FX 8320

Dear Forum Users,

I have a following configuration desktop PC:
Radeon RX-560, AMD FX 8320, 8 GB ram DDR3, 2 disks SDD + HDD.

I have performance issues.
Game slows down to about 10-15 FPS for about 5 to 10 seconds and then back to over 30 FPS and this happens cyclically all of the time. Resolution and display quality does not matter. Tried to go down even to low and 1200x720, does not matter. All maps, all vehicles. When game runs smoothly processor usage is about 35%, when slows it's about 15% only. Graphics card load shows all the time around 100%, but as I said resolution and display quality does not help.

Radeon drivers updated, Windows updated.
Computer is clean, I clean it with compressed air frequently.
Temperature of Graphics is up to 70 celsius.
Radeon settings default, no overclocking needed or done.
I doesn't happen on any other game or app.
Although there are not many demanding games on this PC, rather only things like Roblox and Minecraft.

Has anyone got similar problems?
What might be the source of problem?

Thanks in advance!