Your problem sounds like an electrical failure. Check the obvious first, like loose or dirty battery terminal connections. Then I would check ground connections from the body to the trans. mounting bolts. This is under the drivers side of the car near the steering box. A tired starter is a definate possibiliy and next time it happens wiggle the shifter between Park and Neutral, if this has some effect then a neutral safety switch is the culprit.
The tumbler problem is related to the key not wanting to rotate in the dash. One last thought, the ignition switch could have a dead spot but that would be a unusual thing to happen to your model. Good luck!