nokia 5800 display problem

[sri004]

Well I have this phone for over 2 years now and now I have a big problem with it that won't go away.

The screen goes completely into stripes and is unreadable. It just showed up like that and it's always when I use the screen-lock button (slide button on the side) that this happends and then stays.
Once I got rid of it by rebooting the phone almost 10 times or turning it of a whole day but as soon as I use the screen-lock button (slide button on the side) it comes back.


please help me
thanku advance