problem with Yamaha PSR-172 as midi controller for garageband

Hi,
I connect my Yamaha PSR-172 to my imac with SoundTech Lightsnake. Sometimes a note or synth sound will not stop after I release the key. It goes on and on and on until I select a new instrument or close the garageband file I'm working on. Help would be appreciated.
Thanks,
shirtless