I am not very familiar with AMD CPUs, but I assume that they have some sort of power control. They'll take more power and produce more heat when under a higher load. Old CPUs didn't do this, but then again, they didn't take much power or produce much heat by comparison.

Check the heat-sink temperature with an IR thermometer. Check again for dust buildup. Ensure that your fan is running at the correct speed. If the motherboard is lying about the CPU temperature, it may not be running the fan fast enough.