Hi i guys i have FPS problem and i need help (New PC)

So i buy those items

Ryzen 1500x 8gb Ram MSI b350 Gaming Pro GTX 1070ti

The GTX is not installed yet cause its just arrived todays and i work with ▷ Asus GeForce 8600 GTS

My graphic are set to low....

Btw i instaled windows 10...so maybe that the problem..

So i play the game i start with 250 fps and then i get like 6-10 fps..randomly for like 6 seconds..and then goes up to 100 and then 80 and then back to 9..

Anyone knows why? its the card? its the windows?