BFGMiner and CGMiner use different drivers, you will not likely get BFGMiner to recognize any device with Zadig drivers installed and would possibly have to even delete Zadig before getting the SiliconLabs CP210x USBtoUart drivers to work, not always but some people have had issues with these two coexisting on the same computer. As far as hash rate falling off, make sure to watch what the temps are reporting as well as making sure something else in the house isn't trying to consume your bandwidth from your router.