I have the same problem before, but now with some tweaking and testing in the bios I found the answer. I'm stable now at 4.8GHZ with 1.452v @ 2600MHZ FSB and 2600MHZ HT with the max temp. of 70c at full load with prime95 small fft's
1. set the bios to manual mode on AI tweaker
2. set the desire cpu ratio, (in your case try to put the default clock first) multiplier of 20
3. set manualy the dram frequency to the xmp profile of your memory
4. leave auto the cpu/nb and ht, or incase try to increase both to 2400mhz or 2600mhz
5. disable the amd turbo core tech.
6. set cpu loadline calibration to extreme
7. cpu current capability to 130%, for now leave other in auto
8. set cpu and cpu/nb voltage to manual
9. set cpu voltage 1.4v - 1.45v as a starting point, it is more than enough for the cpu that runs 4.0GHZ as default
10. set dram voltage to the specific xmp voltage in the memory
11. for now leave others in auto
12. disable quiet and cool, c6 state, and all of the cpu power management
13. save and exit..
test in prime95 small fft's in 30min to 1hrs and monitor your temps, if it is stable you can now start bumping some cpu ratio
if not stable pm me and we will try to tweak more in the bios, if you want to check my screen on my rig that runs 4.8GHZ i will post it now
in the forum.
CROSSHAIR V FORMULA-Z = FX8350 @4.8GHZ = 16GB KINGSTON HYPER X PREDATOR 2400MHZ @11-13-13-30T
2 DCUIIOC GTX-660TI SLI = 2 CORSAIR FORCE3 SSD 120GB IN RAID 0 = 1TB WD CAVIAR BLACK
CORSAIR CARBIDE AIR 540 CASE = CM STORM QUICKFIRE PRO KB AND SPAWN MOUSE = 1100W AEROCOOL STRIKE X PSU
ROCCAT KAVE GAMING HEADSET = FULLY CUSTOM WATERCOOLED CPU AND GPU BY EKWATERBLOCK