Intermittent problems with the Command key on MacBook Air

I work with a visually impaired student who uses a MacBook Air with Voiceover. The command key stops carrying out it's normal functions e.g. command tab to move between applications, or command s to save. It is an intermittent problem which sometimes prevents normal typing too, bringing up a series of help menus instead of the letter. Has anyone else experienced this problem and how did you deal with it?
Is there a way of disabling the command key by using different combinations of keys? Many thanks in advance, Pip