Get the HWMonitor from cpuid and monitor the temps.
Sounds like either the CPU or GPU are underclocking themselves at times... likely due to heat. If either are maxing out at >85C - this is the issue.
December 11, 2013 9:38:10 AM
AMD FX-4100 Is at 206C And radeon 7770 is at 48C
Central Processing Unit (CPU)
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I read that as the software isn't getting an accurate reading on the CPU temp - intel has a 105C operating limit (I'd assume AMD has similar limit, I just don't know it offhand) AND as the likely culprit for your slowdows... your PC is underclocking the CPU in order to prevent it from igniting and burning down your house.
First try cleaning the CPU heatsink (if full of dust), if this doesn't help try removing and reapplying new thermal compound between the heatsink and CPU.