The Powe button is flimsy.
I, too have been having these issues. And I think I found the problem - not the solution.
After reading the thread over, and with much mention of the physical button, I decided to take a look myself. After close inspection, I noticed that on the inside of the shell's power button (not the one on the mainboard), there's a little thin piece of plastic connecting the button to the plastic. There are two pieces connecting it - one on the left and one on the right. When new, this allows the button to be pressed equally no matter what part of the button you touched. However, I'm 90% sure the the left piece is cracked, which makes it press at an angle and cause it to stick, which makes the results far from desirable. This makes a lot of sense when thinking of posts talking about playing around with the angle of button presses, cases playing a factor, and even wiggling it around. It's a flimsy piece of plastic!
My advice? Take care of it while you can, and do not wiggle it around in frustration - it may only get worse. At least I know the probable cause, now.... Now to find a fix.