MacBook Pro Boots only with a pressed key I'm having a bizarre issue with my early 2011 Macbook 15'. I grabbed it today without battery power só the first thing I diz was to plug the MagSafe adapter. Afterwards I pressed the power button. It chimed as usual but then I noticed it rebooted by itself after a few seconds then  shutdown for good again. 

I noticed by now pressing the power button had no effect. I tried resetting the SMC and the laptop powered for a few seconds then shut down again. After a few attempts I noticed the laptop remained on as long as I kept a key pressed (any key, like Option).

Since I can't boot by normally pressing the power button I can't do a PRAM reset.

Can anyone help me solve this mistery? 

The laptop was serviced 5 months ago with a new logic board and new Keyboard...
I just found out it seems the power button is "stuck" and pressing option seems to prevent the laptop going into shutdown, as when I booted into El Capitan, a few seconds after I let the option key go, I saw the shutdown dialog for a couple of seconds... I'm going to take it to the repair warranty to recheck the keyboard topcase.

Update: The problem was with a faulty logic board power button circuit. In my case the whole board was replaced as it was still in the warranty period.