I've read a lot of forum pages about this and none of the solutions had worked for me until I've read here the message about the magnet!
I tried some small magnets first but they didn't work. But when I kept the big one(almost as big as a 1 euro coin) around the power button and moved it around there, it worked like a charm!! I opened the phone with the power button and it still works like the first day.
Thanks a lot for sharing that Magnet solution!!