There u go 2 hrs of gaming u encounter no issue at all but just only low fps then confirmed ur gpu die off already

either RMA ur GPU if still under warranty or time to move on and get a new GPU or 2nd hand one
but before that try to use the 2nd PCI E slot and see whether it work for ur GTX 680 to double confirmed if the mobo giving u the same issue instead and i doubt so...and waste of time anyway as u already replaced it
still for me i would get a new GPU covered with
warranty anything wrong u can get exchanged and even if u going to upgrade u can use that GPU back again and u will not get to know which part give u problem u wont solve any of ur problem too...unless u got fren that u can borrow the GPU to test