Question about CPU Throttleing Down.. PLEASE HELP!

I am running a AMD A8 5600k APU at 3.9Ghz...

The APU keeps throttling down after about 5-10 min under load.. it goes from 3.9Ghz to 1.3 and the voltage drops from 1.43V to about 0.374V. The temps when this happens is about 61C and it has never happened before.... PLEASE HELP

SYSTEM SPECS

APU- AMD A8 5600k
GPU- GTX 750
MEM- 6gb
OS- WIN 7
MOBO- BIOSTAR A55ML2