The problem might be due to a worn out clutch master cylinder.
This component is positioned under the hood. When it's cold, the seals in the master cylinder can contract where the proper hydraulic pressure isn't produced to operate the clutch. As the engine heats up, and the underhood temperatures increase, it heats up the master cylinder where the seals expand. Hydraulic pressure is then restored, and the clutch functions normally.
I'd be looking at the clutch master cylinder for internal leaking when it's cold.