It seems as your whole power system (cable to PC) is not able to get that much power, so that voltage crashes and with it the PC. How does the PC crash appear? does it reboot?
It just goes dead and reboots, as if the power had gone out in my house and been restored.
I think my next best option is to attempt to get an adapter for the 8 pin power cables. I've never seen an 8-pin to 6-pin adapter though. I've seen plenty of 6-pin to 8-pins.
An 8-pin should be able to plug into a 6-pin slot, however these connectors have extra plastic that prevents that.