Hello. I have an issue on GTA V and Rise of Tomb Raider.

After 20-30 ( very random ) minutes of non activity ( not touching mouse or keyboard ) in games,
I have fps drop and gpu core clock for a fraction of a second and then bring clocks back again.

Screen during drop, i added black arrows to show:

It drop for a second from 1985mhz to 1759mhz. UTIL gpuz. GPU LOAD 99% .What can it be? System clean, all programs closed, no spyware or anything weird in task manager. Windows 10 Anniversary. Newest drivers. Also i tried with 2 previous driver versions,didnt help.

PC: 6700K No Oc ( good temps )
Gigabyte Xtreme 1080 ( good temps stable )
Corsair 750 RM
Asus Z170-P SSD
1x 256gb Crucial

Power settings: Windows Power Settings: Balanced

Nvidia Power Management: Optimal

All power saver options OFF. Monitor turn off: Never, Sleep,Hibernate: Never

Any ideas? Is this related to Optimal power ? Any clues?