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Are you running 3 gpus???
This MB shares bandwith i belive
1 card will run ar 16
2 cards will both run at 8
3 cards will all run at x4
If you are running just one is it in the top slot?
try cleanig the pcie slot on your motherboard of dust,etc,this happend to me a few days ago,after the first clean it went up to x8 then i cleaned it again and its back at x16