I have heard of this twice ,both times the PC had no visable faults that could have caused it .one of them was a pc I had built and it was brought back for me to look at . although I never got to the bottom of it ,I did notice that the date it went back to was the same date and time of the first restore point in system restore .and that system restore was just about ful .but I cannot say if that played a part . I switched system restore off and back on to reset it as the PC had ben performing perfectly apart from that one Oddity . it never happened again .
I cannot say with any certainty that it was releted to system restore but in the absence of any real evidence ,I am keeping an open mind . and fingers crossed.