737 elevator trim problem /FS9

Hi guys,While flying 737 I'm experiencing very strange elevator trim behaviour. When pressing button assigned to trim up or down the control surface moves a bit and after about half a second returns to the neutral position. Think I dont have to add how annoying flying untrimmed aircraft could be. I've recorded video to help explain my problem:http://www.youtube.com/watch?v=_2_SpfxvSA0(trim button constantly pressed)Any ideas? Problem occurs even on newly installed Windows and FS, all settings are default, all updates installed.Let me know if u need more details about my hardware or something.Thanks in advantage!Bartek.