It sounds like the clutch master cylinder starts to leak internally once it gets hot. The master cylinder is mounted on the firewall in the engine compartment. As the engine gets hot, it heats up the master cylinder. The bore in the master cylinder expands from the heat, and hydraulic fluid bypasses the cup seals in the master cylinder where no hydraulic can be produced.
To confirm this, the next time the clutch hydraulic pressure is lost, pull the vehicle over, open the hood, and pour cold water over the master cylinder. If the hydraulic pressure is restored after doing this, you know what the problem is.
Tester