I got the exact same thing on one button - fortunately its on the p1 start so not an issue for me. I exchanged microswitches to check for a problem there, no change in behavior. I checked continuity on the wire from the switch to the ipac, no prob there, but didn't go as far as replacing it.
In notepad, it's exactly like you say. every other button shows up as fast as I can press them, but that one registers one letter for about every 2 presses. I can't remember what letter it is but it's p1 start on the opti pac 2. as best I can tell it does send a keypress the first time it is pressed.
Any ideas? I wouldn't even have posted this but it's interesting the same issue arose with another cp. I bought the Ipac in march 2011. Can get serial #'s etc. if needed.