1998 Subaru Legacy car wont start

My son drives a Subaru Legacy GT auto with a 2.5 engune. The car runs and drive fine on a dialy basis but about once a week, he puts the key in and all the dash lights and sounds come on as normal but the ignition will not fire at all. It's completely dead. However, if he leaves it sit for a while and tries to start it up, it fires up just fine. The battery and charging system have been checked out (both fine), the terminals are clean and all the connections are tight and a new starter was installed about six months ago. This problem started about six weeks ago and seems to be reoccuring more and more often. Someone suggested some kind of a faulty switch or sensor related to the the shifter and something not engageing when you put the rig in park? Any help would be appreciated.