I'm really not sure what to do since I'm not sure what could be causing that. First I would try plugging the keyboard into different jacks, then I would try a regular (old-school keyboard), then I would try to reinstall Mint since it's possible the installation went bad.
P5N-E SLI, Core2Duo E6850 3GHZ X 2
2 GB DDR2 800MHZ
EVGA 8800GTS 320MB
Arctic Cooling Freezer 7 Pro