ASRock Z87 EXTREME6

I can play Dota 2 for 5 minutes, then my FPS start decreasing from 105 - under 90 when i use B (Hotkey for my shop) then it will drop down when i close the shop it will go slowly back to 105 and stay there until i come in a big teamfight, when at lot is happening on my screen, it can even drop to a 67 FPS.

if i cap my FPS at 200 it will still likely to run down to under 100 if i open shop again..

I am thinking can my CPU, GPU do this? i don't lag in Dead by daylight nor i drop down in FPS there.

Don't think Dead by Daylight runs on Source2 engine as well.

ASRock Z87 EXTREME6

i5 4670K -> Seidon 120M Cooler Master

ASUS Rog Strix 1070 OC Editio

750Watt Fully modular PSU Corsair

16GiGs 1866 -> Locked at 1600 cause MB

120GB SSD Evo 840

500GB (3-5 year old HDD) numbers: WDC WD5000AAKX-08CA1

Nothing is currently OCED or anything

Upgraded BIOS from 2.10 - 2.40

Thank you, from this it is very clear you have a problem  Without this info all I could see were the perfectly playable frame rates you were mentioning 

Playable as they may be they are far from what this system should be producing. I have some further questions that should help pin down the issue.

1. Is your RAM 2x 8gb or 4x 4gb?

2. What are your system temps like? (there are numerous temp monitoring apps out there)

3. While you are running a game (and it is giving you lower than expected FPS) run CPUz and let us know what the "Core Speed" value is.

4. Have you tried clearing CMOS on your system? If not do so.

5. Have you installed the latest "Intel Management Engine" drivers?

Another useful thing you can try is download GPUz and run it before you open a game, leave it running throughout your gaming session. Once you have observed the FPS issue you are talking about, quit the game and look at the sensor tab in GPUz. It should have logged a lot of info about what your GPU was doing during your play session. Click on the down arrow next to "GPU Core Clock" and select "show highest reading". The number shown should be the maximum boost speed listed on your GPU's specifications. If it is significantly lower (300mhz or more) then the issue is GPU/driver related. If it shows the correct value then we need to look at the rest of your system.