What is intermittently faulty and leaves no fault code? Ignition switch? ECU?

One of my daughters has an unsafe 1992 700 wagon; Rex Regina B230F. No fault codes stored. Without warning the engine cuts out. Occurs without apparent stalling, sputtering or misfiring; she could be driving 45 MPH or coasting slowly, or, coming to an uphill stop.

Full tank of gas. Miscellaneous. Intermittent. The engine may be able to restart immediately or after a few minutes, or, days later having been towed home. It might start right up and run perfectly for a couple of weeks. I'll mistakenly think that something I unknowingly "fixed" was the culprit this time (fuel pump relay, loose coil wire, bad ground, dirty throttle body, dirty idle air valve, RPM sensor) and then it cuts out again.

She drives it to my house and I find no clues and start looking again. This is not a safe vehicle. When it won't start and evidently nothing I did makes it start and run perfectly again after repeated attempts... it is puzzling. I need to nail this one down.