Car randomly loses electrical power completely

Hi, I have a 99 nissan altima, and afew days ago while stopped at a traffic light the car just turned off without any warning. No power what so ever, light did not turn on and turning the key did nothing. A few minutes later however the power was back to the dashboard lights and I was able to turn the car back on with no problems. The car died again the next day while stopped on the road but I was able to start back right away.

now the car is parked at home and yet again it has lost all electrical power. I tried to jump it and nothing happened, the battery was showing 13.7v or so but the lights would not turn on, and the car would not beep when the key was inserted in the ignition. The radio however would kick in every once in a while when I insert the key in and turn it, but would not necessarily turn on again when if I pulled the key in and out. (This is all while the car is being jumped) I removed the jumping cables from the car and still nothing. However a few minutes later I put the key in and everything is normal and the car starts right up and shows the same 13.7v on the battery (12.5v or so when the car is off)

So I turn it off again, pull the key out and insert it back and again its completely dead even though the battery is showing the same 12.5v. Again a couple of more attempts and the power is back.
when the power is cut, turning the key in the ignition does not do anything and no light come on the dash and head lights will not turn on, its as if the battery wasnt even installed in the car. I have checked the battery connections and they are in good shape.


Any idea what is causing this?