the engine shuts off on the highway at high speed
If it shuts off as if someone turned the ignition switch off?
Crankshaft Position Sensor.
Another idea, faulty ignition switch. Try driving with just a single key in the ignition, not on a chain of keys. If that helps the ignition switch is probably the culprit. Dangling keys can swing when going around corners and over bumps & eventually damage it.