That's throttling and definitely the motherboard. There's a few things you can try like pointing a fan on the power section and look for APM and disable it in BIOS ( don't think it'll have the option) but in reality the board can't handle the CPU
The temperature you quote us is the processor temp, I'm guessing. Those are borderline for an AMD CPU but probably would be higher except for the throttling that is keeping them cooler than they might be otherwise. If you were to look at the temps of the VRM (power producing components, "Voltage Regulation Module") that's what's getting too hot and causing throttling.
Had that same motherboard it was pain in ***. Could go to 4.0 but would do the same thing meaning its useless OC. I don't suggest doing any OC on it, that motherboard simply cant handle anything.
On what you should be looking for are tmps of Package, socket and Motherboard in general, other things are speed drops and vcore drops. That motherboard dosnt have LLC meaning it will probably receive pounding on clock speed and vcore I think you already have that problem. Any drop larger than 100mhz is alarming and have to be reduced its clear indication that it isn't stable. In my situation LLC solved that little issue