I've had EXACTLY the same problem on the same year. Most of the time it happened on road trips where we'd listen to the radio while resting, only to find the starter no longer engaged when we were ready to get back on the road. First I thought that the battery ran down. Screwdriver at the solenoid to get going. Then it was happening every few weeks. It eventually happened at home one day, in nice weather, while I was not in a big rush. Got out a voltmemter to diagnose.

In start position... there was more than 11 volts after the interlock, but only 8 at the starter solenoid. Alarm system was wired before the interlock and had 11.8 volts on both side if its relay. I traced the voltage loss to the wiring section between the zillion pin connector at the left door post and the corresponding pin of the 8pin plug to the engine harness.

Instead of trying to run a new wire through the firewall or cutting the big harness open, I added a relay underhood. It is physically tied under the crossbrace above the master cylinder. Original green wire (shortened) to pin #85, and added ground wire to #86. Using 16 gauge wire: #30 to battery stud of starter, #87 to the push on blade of the solenoid.

Before rushing off to do this, elaborate on what the dash lights do when you turn the key to start. If they stay on, then you have a similar or identical problem to mine... current loss in the wiring between the neutral interlock and the starter solenoid. If the lights go off - ignition switch or power feed to it is flaky.

Mine's been 100% dependable since that modification more than a year ago.