Does the 91 have the 3 digit codes? I thought it was 93 and up?
Next time it does it try this:
Key on, crank, no start.
Key off.
Key on, Crank, no start.
Key off.
Key On, Crank, no start.
Do that a few times and see if it starts. If it does start, then it would be the Cam sensor. With the computer not able to see the cam sensor, it will use the crank sensor and guess. It guesses what cylinder to fire each time you key off, and back on. Eventually it will get it right and the engine will start.
Since it's an auto, you can verify the Crank Sensor function during no start by watching for the Upshift light when the engine is cranking. Normall it should be off. If it's on, that is a signal that the ECM is not receiving a signal from the Crank Sensor.
I'm thinking Cam sensor... but if it totally won't start no matter how many times you do the above process, then it's not cam. Then it might be the crank sensor, or wiring to the crank sensor.
Don't forget to do the #1 test on our SC. Make sure the bolt is still retaining the Harmonic balancer on the car! And with the car running make sure the big pulley on the crank is not wobbling. If you are missing the bolt, or the pulley wobbles, the harmonic balancer needs to be replaced.