Do you use Windows or Linux?
With Linux, it's quite difficult to overvolt your GPUs, you'll need to flash the GPU BIOS (see my sig). On Windows, there are dozens of manuals floating around, just google it or scroll through my guide, it has a link to one of them somewhere in it.
That the crashing occurs later means that your GPU is running at it's limit and since load aloways fluctuates a little you are pushed over the limit randomly. And at one time it's too much for your GPU to handle and it freezes.
I am familiar with both operating system platforms.
I edit my bios files using RBE in windows, flash them to the cards using rbe or atiflash, and then mine using linux...
Are you saying that my GPU is running at it's "electrical" limit and I could possibly increase that?
Basically I am an overclocking newb who is comfortable playing with core/memory clocks but am unsure as to what will allow me to increase my core clock to a higher speed and keep it stable. If the temperature is ok... power seems to be the only inhibiting factor in my mind no?