First check and see if the discs you are using are even capable of burning @ 8x.
Check the SmartBurn area of my Tools Guide (link in my sig). SmartBurn will tell you the maximum speed your discs will burn in your drive.
Edit: If you are using -R discs then you can't burn them @ 8x. The 851S only supports 8x writing on 8x +R capable media (including a few ones that are sold as 4x +R).