My iMac 27" Lion has an intermittent problem where it seems like the space bar is sticking, but the keyboard checks fine

Hi - I got this refurbed iMac from the Apple store summer of 2011 and lately if has had a problem where every now and then it will show symptoms of a stuck space bar on the keyboard in that it will enter lots of spaces in between any characters I type and if I am in email if will jump to the top email in the list no matter what I try to select. I took th ekeyboard to be checked at eh Apple store and they said it checks fine, so it is something internal inthe computer / software. I am running OSX 10.7.3 and this is a wired keyboard with the number pad. I also have a wireless mouse that I have tried swapping batteries out in as well.

I found something about a SMC reset and a PRAM reset possibly being the problem and I do those resets but it is still doing it, and so I have been having to restart whenever it happens and then it is OK for a while. I don't know if it's coincidence or not, but this started after I got an Apple wireless Touch Pad which I later decided I didn't like and returned - at teh Apple store they said they didn't think that there could be any kind of conflict going on with that and the mouse but the Touch Pad has been returned and I still have this problem.

Any ideas? They said to bring it in for a diagnostic at the Apple store which I will do if no one here has any solutions. - Doug