power button malfunction

I have a xperia M C1905 which has a weird problem with the power button. When the phone is turned off I press the button and boots up normaly.After that there is no response.No switch off or reboot menu not even stand by (screen goes black). After awille the the phone goes to stand by and no matter how many timew I press it there no responce.I have to take off the battery and power the phone. I did a factory reset and flashing stock rom again but nothnig changed.
Software or hardware issue? Any ideas?