In this scenario (AMD A10-6800K 4-core, MSI 2AE0 mobo, 20GB RAM, Win10 Pro, AMD O/D 4.3.1.0698), I turn OFF Turbo Core Control, and start a Folding@Home 3-CPU work unit. The CPU Status pane shows 4100MHz, 41x multiplier, 1.3250 CPU VID and 20-22°C thermal margin on all cores, and all is happy. The machine is otherwise lightly loaded, in particular the GPU. Later, for some reason, AMD O cuts the frequency to 1996MHz, 20x, 0.9125 CPU VID and 40-42°C margin. This halves the throughput on the FAH work unit. I note that the GPU clock is unchanged if I add a GPU work unit.
I don't think this is heat-related throttling as the fan never speeds up and the machine load stays low and constant. In the Clock/Voltage pane, the CPU Core x Multiplier slider stays at 41x, and moving it does not change the clock speed. The CPU VID slider is unchanged as well. To get back the standard clock requires rebooting.
I never thought a feature of AMD Overdrive was to slow down my machine; maybe I should call it AMD GrannyGear. Thanks for any ideas.