My MacBook Pro (running Leopard 10.5.8) won’t allow keyboard to type an upper case 'C' using the shift key...works fine with caps lock, or, with my Typinator workaround using double-typed lower case c (not always best). Any ideas?

My MacBook Pro keyboard won't type an upper case 'C' using the shift key... only with caps lock. Workaround has been to use Typinator by typing a double lower case c, not always the best solution. Mac is a refurbished model, which initially was fine. Apple tech helped me correct the quirk when it first appeared, but now it has returned and refuses fixes. Any ideas?