The clutch master cylinder is being effected by heat.
When the master cyilnder heats up it causes the bore within the master cylinder to expand. If the cup seals in the master cylinder are at all worn, the master cylinder can no longer produce the proper hydraulic pressure to fully disengage the clutch. This would be like trying to shift into any gear without stepping on the clutch pedal.
We just had another poster on this board complain that they were replacing the clutch on their Mustang every year. You can also burn up a clutch if the master cylinder gets effected by heat.
Tester