The ignition switch (electrical portion) is known to get 'tired' after 45 or so years and do intermittent things like you describe; also the internal spring gets tired.... When replacing an ignition switch, its best to deal with a vendor or someone that is intimate with the NINE POSSIBLE 'Pantera' switches you might find. Some will plug into the stock wiring loom(s) while others have extra wires or missing ones, wrong color codes. A few complete switches will not bolt onto the stock steering column, or are too short to protrude from the plastic covers. All in all, if it IS the switch, it may be smarter to send it in to an expert for a rebuild. Note- that will NOT be me!
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