Restarting PC

[KaiWachiX]

Hello guys,

Few days ago i upgraded my GPU from GTX680 to 1080Ti.
My first problem was when i plugged in the new card into the first PCI E slot its not working, so i need to use the x8 pci e slot, whats work, but GPU Z says its only at x4.
Next problem, when i play any games, after 10 minutes my PC restarst itself, and i dont know why. I cleaned out the PSU, put another fan inside the case, and its still happening. Im really dissapointed right now, because i cant play nothing and spent a lots of money.
Whats weird is when i ran AIDA64 for 30-60 mins everything was fine without restarting.
my Spec: i7 3770k
GPU: Gigabyte Waterforce 1080Ti
Samsung evo ssd 500GB
4x4 Corsair vengeance 16gb DDR3 memory
PSU: Corsair HX850
MOBO: Gigabyte Z77X-UD5H
CPU COOLER: Corsair H110i GT
I hope u can help my problem, 
Thank you!

[KaiWachiX]

thank you....