wireless keyboard volume not working

I have iMAC OS X 10.6 since early 2010 and haven't had a problem with it ..until about 3 months ago. Then suddenly my volume control on my wireless keyboard stopped working. Every other keys works perfectly. Now I have to manually control the volume, (using my mouse) from the Preference page.

I've checked the forums and I know this is a known problem. I have tried all advices given to others. I've tried rebooting, pressing "default" on preference, changed batteries, disconnected from imac bluetooth, unpaired it THEN re-paired it. Turning my imac OFF then ON ... it'll work for an hour max, then goes back to being unresponsive. I've even brought my keyboard to Apple store, they couldn't find any fault with it. I only use this imac for editing purposes. The only softwares I download are Apple updates. I don't know what to do. help ?