BeagleBone powerup problem

I got a BeagleBone. But recently the board have some problem with
powering up:
1.puls into USB,power up successfully,system boot up!
2.Pull out USB,then puls into again(one or more times), power LED is
off,the board cannot be powered up.

USB(pin 12) :5V
SYS1(pin 7) and SYS2(pin 8) : 0.7~0.8V

3.Pull out USB,wait for one or more hours......
4.puls into USB,Maybe power up successfully again~~~ But Pull out USB,
then puls into again(one or more times), the board cannot be powered
up again.

Please Help me , tell me where problem is~~~~

After you plug inthe second time, what happens if you hit the reset button on the BeagleBone?