Which parts of my PC is bottlenecking or causing this problem?

[bottlenecks]

I have a problem. I have a gaming PC and my games run very fast at first then run very slow and lag a little. I think I could be bottlenecking because I am running

i7 4790k @4.00GHZ
Gigabyte 970 G1 Gaming SLI
Gigabyte Z97X UD5H Motherboard
CS 750 M Corsair Power Supply
16 DDR3 Memory
Windows 10

and I removes my other GPU from my MoBo and it ran faster and took a longer time to starts lagging and it lags less frequently, problem is I don't know what is causing the bottleneck (if it is one)
Please help all products are new

 

[Greantea]

I think its your psu, look at the psu tier list. Yours is at 3rd
Your on sli which required more power and a good qualyity psu

 

[KNARF XD]

I think its your psu, look at the psu tier list. Yours is at 3rd
Your on sli which required more power and a good qualyity psu

yep, probably the psu

 

[bottlenecks]

I think its your psu, look at the psu tier list. Yours is at 3rd
Your on sli which required more power and a good qualyity psu

But on pcpartpicker it said that it was 550 Watts or something with SLI. Does it not count gaming?