This video describes pretty same problem:
http://www.youtube.com/watch?v=3QpNl42RO6c
According to comments, this bug can be induced
1. By dirt on the back side of the touch screen
2. By broken digitizer
So, please tell me.
I got to go to the service center or i got to try cleaning up the screen?
If someone already cleaned up screen, please give me some links on good manuals.