Power malfunction

I have had my BMCC for a few days now, and a power malfunction (I don't know how else to call it) has come up. At random (or with pattern that I have not yet managed to figure out) the camera would simply not start on internal battery, even though fully charged. It would eventually turn on after inserting the external power supply, again after refusing to turn on for a while (i've had instances when this would happen for 5 minutes or half hour)... Has anyone encountered this problem? Any solutions
Thanks
Thanks