GPU, Motherboard or PSU?

[BubbleTrouble]

I have a problem with 1 of listed things.

 

GPU: GTX 970 G1 Gaming Gigabyte

Motherboard: Gigabyte Z97P-D3

PSU: 620W Seasonic S12II

 

The issue is that my PC won't turn on properly from time to time.

 

What it will do is: 1. Get power, spin all the fan to max and keep them that way forever. So I have to press power button and it turns the PC off.

 

Next time I turn it on it spins fan to max, boots, fans are normal, I get to the windows.

 

Then, my GTX 970 will work weak. Then I have to restart PC couple of times and it is back to normal speed.

 

What I mean weak and normal. For example: Unigine normal = 40fps everything max first scene. Weak = 14-18 FPS everything max first scene.

 

I have BIOS settings on default. Did not have these kind of problems before. Also CPU works fine.

 

So now I'm wondering is it the motherboard fault, GPU or PSU?

 

I'm thinking of motherboard as everything works fine when it does. And it is not always. So I'm wondering if it has something to do with voltage regulators on MB?

 

I don't have any spare parts to test it without them.