I thought you said you motherboard was slow, until I re-read this.
Anyway, the lower cpu mhz is due to a mechanism in AMD CPUs that slows the speed down until needed, saving electricity.
As to why it is slowing, it could be many a things.
Try the following software:
Ccleaner - this will clean up unused files, but won't hurt any program you may use.
Malwarebytes - This will check for registry errors, and fix them
Also, go to run, and type msconfig
Go to the startup tab, you can able or disable any program that runs at startup. That can potentially slow it down.