The problem is that the contacts are dirty or damaged. Take apart the lo-profile case or the hi-profile keyboard enclosure and remove the keyboard assembly. Remove all the small phillips screws attaching the base of the keyboard. Cut the two nylon ties holding the hi-profile keyboard cable (RadioShack has replacements). Unsolder the two connections for the shift lock key and remove the bottom cover. Clean all the contacts on the bottom cover with RadioShack contact cleaner, blow out the whole assembly with canned compressed air. If you notice any wearing of the gold plate on the contacts repair it using conductive paint. Take this opportunity to clean dust/debris from under the keycaps themselves too. This should fix the problem.