Seems like this vintage of Toyota lock cylinders wear and develop the problem you describe...guessing roughly the same core cylinder part used on a number of Toyota models..

My wife's 88 Camry did same thing, have read of other Toyotas doing same..

Replaced lock core, which means you now have a new ignition key to carry different from the doors <img border="0" title="" alt="[Mad]" src="images/icons/mad.gif" /> <img border="0" title="" alt="[Mad]" src="images/icons/mad.gif" />

No problems since replacing core..