swell9 :
RealTemp reports ~50 during prime95 blend test. So I don't think temps are a problem here.
What's the problem then?
Memtest => passes => no memory error
small FFT => passes => no CPU error
could it be a defective motherboard? Although I see myself being really really unlucky to have purchase a defective mobo by chance.
If you are stressing this system over an hour with prime95, your components are almost certainly fine, it's just fine tuning the system.
Take a gander at the posts and instructions on using prime95, like setting the advanced round off checking, etc.
Take a look in your BIOS at the Vcore voltage (choices up to where gigabyte lists it as Red). I think but am not positive the Vcore of your CPU can go to a max of 1.375V. Might increase the Vcore just one notch and see what happens. Remember the Power (Heat) increases by the square of the Voltage, so increasing the Vcore will increase the heat generated, but your Max heat is about 73 degrees, so there is some buffer.
For a stable system you should also check to make sure your UnCore mulitplier is twice the multiplier fo the RAM. That may be the issue since you upped the RAM multiplier.
Maximum PC has a great article on i7 Overclocking, but the concepts and components are the same here.
When you stress the whole system, parts include the CPU cores, Mem controller, QPI, PLL, etc. (the components that make up the i3 CPU.
Some people only prime95 test their systems a much shorter time, so you are getting close.