If you could give the exact model of the graphics card that would help, but to me 750Watts running graphics two graphics card at full ( which is what any kind of crypto mining does ) is the bare minimum for your energy requirements, it may be shutting off one down so that you can keep the other running.

It would be baffling if you were running low end/mid range graphics cards but high end graphics cards would explain your problem, I have a high end graphics card ( ATI Radeon 6970 MSI Lightning ) running all night and periodically in the day and I have had no problems with a 700 odd watt power supply.