Put the key in the ignition and rotate from off to run 3 times. You should hear an audible ding.
If you don't hear the ding repeat until you do.
What this will do is reprogram the key, or should.
With aging cars it seems to be becoming more common for the keys to get weak. There is a chip in the key and a RFID receiver around the ignition cylinder. Syncing the signals may help, one may be drifting out of range (frequency) of the other with time.
If that works, you're welcome. If not I'm sure I can come up with more ideas.