I have a 2009 MacBook Pro 17" running OS-X 10.10.4. Then a while ago it showed a new behaviour: when I unplug the power after a few seconds (3-5 or so) it abruptly turns off. When I try to start it without the power plugged in, it turns off somewhere in the middle of the startup process (grey screen with Apple logo and a progress bar). The curious thing is though, that when I start the MPB without the power plugged in and I then press the ALT key for the boot medium selection then this state is shown for hours without a problem. So it's not the battery (I would say). What else can cause such a strange behaviour? Oh and one other thing: when I try to boot the MBP WITH the power plugged in sometimes it takes two to four tries until it finally really starts up. On the other tries it just makes this little startup noise and then nothing. Really really strange.