Seeing as you used the phrase "Sleep" and made no mention of knowing that the actual poweroff function existed (asking for it to be in the menus, instead of saying "The power off button does not work for me" letting us know you were experiencing a real problem other than not reading the manual) we could not possibly have guessed you'd tried that.
As well, try installing Rockbox the official way, using the official bootloader, and see if that resolves your button problems, as for strange reasons it has in the past.