weird keyboard problem?

I just bought a new wireless keyboard from mac a few weeks ago and when i connect it to my imac (10.6) it works perfectly the first time but then each time after the down arrow continually presses down. Does anyone know what could be wrong?
I took it to get checked out and they ran diagnostics on the keyboard but could find nothing so I'm thinking it might be something to do with my computer.
Any help would be great!