Talan,

From what i read there. (System freezing)

You have a power issue.

I've got over 100 Nano's, there's isn't a single one that can't handle 1000 core @ -100mV VDDC.

Obviously 800 core will run fine, it consumes practically nothing :p

If you raise your core clock, your power usage will go up. And not by a little, it will go up a lot if you compare it to 800 core clock.

So again, you're experiencing power issues.

How many nano's was that? 2, 3?
What kind of power supply do you use?
How did you connect your PCI power cables to your PSU? (2 on the same rail can cause it to crash..)
Have you tried just running 1 (1 connected to your motherboard "without a riser")?
What kind of motherboard is that? H81 pro btc? Molex connectors connected to the motherboard if you use non powered risers? Or if they're powered risers, than don't connect those molex connectors..

I can sum up so many things..

you'll have to figure out where it's going wrong mate..

Greetings!