Well if it is a 6+2 that end should be in the GPU. If both ends are just 8 pins I sure hope the end say PSU or GPU.
I would say if its been working for half a year its wired right.
So I would take note of which red light is on now, turn off the system and swap two connectors the one with the red light and one on other wire, ( This is at the GPU only) and see if the red light moves with the cable swap. If it does it may be a bad cable or PSU issue, if the red light stays where it is may be an issue with the GPU.
If it looks like a bad GPU call EVGA and do some troubleshooting with them and maybe a RMA.