Given the age and intermittent nature of the issue, I would suspect a short to ground somewhere in the harnesses. These are difficult to find, especially if temperature related expansion is what leads to the short.

Given that, and in general if this is a keeper, it may be a good opportunity to insulate all harnesses and wiring that runs on, over, near the engine and chassis in the engine bay. Pay attention to any areas with a high heat load, near exhaust etc. And the component connectors.

If that doesn't solve it, you could also have fractured/weak solder connections in some of the key relays or ECU. Usually these will be at the pins to the board where the harness plugs into. There are many videos on how to both here and youtube.

It's a lengthy process to sort out, but at least in going thru the process you can also eliminate potential sources in the process.

Good luck