The symptom I am about to describe is happening on two computers, both MSI K9N-Neo-V3 and AMD Athlon X2 4000+ and AMD Athlon X2 6000+.

Regardless of BIOS settings, overclocked or on stock speeds, even after a straight CMOS reset, and trying two different BIOS versions, 2.2 and 2.6. Ramp up the CPU (both cores) , after a few seconds the temperature reaches 65C and then as soon as it hits 66 C the multiplier drops from 15x or 10.5x to 5x. The temperature recovers and the multiplier goes back to 10.5x or 15x.

Thinking this was simply an artifact of CPU-z I ran my own CPU speed tests and sure enough they agree with CPU-z.

I am puzzled as to what's causing this - do not think it is the CPU. Must be the motherboard. Both computer cases have very good ventilation and fans and the ambient temperature is low. I have re-sitted the one CPU (the 4000+) and applied new thermal paste and made sure there was a very good contact with the cooler. The coolers are stock provided by AMD (they are not small, they are quite bulky). It did not make a difference. I then tried to lower the voltage to the 4000+ which the "AUTO" BIOS setting has it at 1.29V - I lowered it to 1.2V but became unstable.

Please note I am not overclocking, everything is running at stock speeds.

If anyone knows anything about this please let me know.