Is this an authentic 5,1 or a 4,1>5,1?
What firmware (boot rom) are you on?
I'd suggest replacing the 3V coin battery on the board. This is BR2032 as OEM. The CR2032 batteries work and are usually easier to find locally at retail locations, but they do not last as long. This is a very cheap and easy replacement to begin to troubleshoot the issue.