Many people have this problem, i think they all clock ther nvidia cards to high. I had the same problem in the past, makes no errors for a couple of days and then restarts with the same error. I clock 1 of my cards lower, now it works perfect.
The problem is, on nvidia cards we can not see hardware errors in hwinfo like we can see it on amd. this is my opinion and i working with over 100 gpu´s.

I see you have 22 incorrect shares on gpu 6, another indication of excessive overclocking.