weird keyboard problem

[bellend]

hi there - i recently lost my blackberry torch and went back to using my old 8900. Everything was fine for a few weeks, but now im having a strange problem with my keyboard / buttons.
whenever i stop using the phone for about 30 seconds, none of the buttons on the phone work, i have to hold down the blackberry button to make anything work again, also the space bar on the phone doesnt work either. This has happened before and after taking it apart about 20 times it eventually fixed itself. I have no idea what i did to fix it originally, but the problem has returned.
Anyone got any ideas on what is causing this problem + how to fix it?