Same thing has been happening to me since Jelly Bean update last year(beginning of 2012) and yesterday my power button just completely stopped working(really annoying!)
I found an easy way to use the volume button to turn on the screen without the fix-it button app(I think that's what it's called)
1. go to settings
2. click on security and remove your screen lock(or else it doesn't work)
3. go back to settings
4. click on display
5. click on sleep
6. decide which option on sleep you want(15sec.,30sec.,1min.....)
7. when your screen becomes black(goes to "sleep") press on the volume button and there you go
I hope this helps:)