If the CPU was under load when you looked then it may have been thermal throttling to 2.9GHz to lower temperatures, if so then invest in an aftermarket cooler.
If you were just sitting on the desktop doing nothing then the CPU probably just down clocked it's self to save on power and it'll ramp back up again when it need the extra speed, this is Cool n' Quiet a dynamic core speed technology for AMD CPU's.
If neither of those are the ace then you may have misconfigured something in the BIOS or UEFI and you should double check that both the FSB and Multipliers are set the the correct settings (FSB = 200 and Multiplier = 17.5) or just set them to "Auto" or "Standard"