I had a very similar problem with my '91 and it turned out to be a loose connection in the wiring harness under the dash, just in front of the drivers door. I found it by holding the key in start while pushing/pulling different wiring. The white plastic connector had started to melt which was the burning smell. Next time it happens, feel those connectors for heat, then try moving them with the key in start. I ended up cutting the connector out and soldering the wires. You might even see the blackened plastic where the arcing is occurring by giving it a close inspection. Good luck!