This problem has been around for a while. Common troubleshooting to be done is to:
1. Download the latest GPU-Z/ CPU-Z and check to see they both report the same thing.
2. Use latest drivers. If in doubt, do a fresh install of your driviers using
DriverSweeper. The correct usage of Driversweeper is to a) uninstall your existing driver using the Control panel and restart. b) Upon bootup, run Driversweeper ticking the ATI/NV display driver box, then restart. c) upon entering windows for the 3rd time, double click the executable of the new drivers and follow the prompts. This method limits conflicts, as some files are not entirely removed until a restart so if you were to prematurely install the new drivers before a restart there is the potential for conflicting files (some files having been cleaned, but still being present in the registry) anyway moving on.
3. PCI-E frequency in the BIOS. Some people have found increasing the PCI-E frequency by as much as 1mhz (to 101mhz) is enough for it 'switch over' into 16X mode. Try anywhere from 100-105mhz is safe for these purposes.