5.0 on air...you sure about that???
Even without OC...you might try increasing the cpu voltage a bit more...say 1.36v the actual stock voltage and see if it stabilizes it. Bump it a bit more if necessary. You won't hurt anything as long as temps are monitored and you don't exceed the maximum limit for that cpu.
There is likely a whole section in tfm for your motherboard dealing with settings and power management for overclocking beyond just cpu voltage...
Is the RAM OC'd from the Turbo Boost/BIOS or did you manually configure it in BIOS? Most sources would recommend little or no direct adjustment of the RAM as you can introduce many stability problems by fiddling with it.
Are you using one stick or two for your RAM? They need to match.
Readers may benefit from the recommendation that if you are going to Crossfire...make it worthwhile...replace a modest performing card with either a single faster, more powerful card or two faster, more powerful cards. Avoid adding another weak card to the build.
02:24 AM by