Dell E7440 memory issue

Hi,

I upgraded 6 months ago my Latitude E7440 with new memory :

- 2 of those => http://www.crucial.com/usa/en/ct102464bf160b

And here my problems start ..

Sometimes, my PC won't boot. It just show a blank power led and nothing appear on the screen and then after 10 to 15 seconds the blank power led shut light off.

And the only way to solve this problem is to unplug the battery on the motherboard to clear the BIOS.

Only then, when i plug the power source the laptop boot automatically one time and show nothing on screen, then reboot and show nothing, reboot again and i get access to the bios and everything ...

It's very strange because sometimes it appears randomly and sometimes it appears exactly every seven days (3 times in a row !).

Of course I tested the memory and I got no error.