Try spraying the key with some tuner cleaner or wd40 and rock the key back and forth a little when you push on it? Oh yeah,be careful not too drench the key pad so it doesn't drain on to the pc boards!:wink:
If memory serves me, the keyboard contacts on the 210XLT are a thin printed circuit film enclosed in a plastic membrane. As such, spraying contact cleaner on the switches won't have any effect on the switch contacts. Doing so may help if the hard plastic #5 keyboard button is gummed up and sticking.
If the actual underlying film contacts are worn thru, you'll need to look for a junker 210XLT on eBay that you can rob the part from.