In my opinion, it could be the clutch cable. When metal heats up it expands. Maybe your clutch cable is expanding just enough to make it ineffective in disengaging the clutch.
Your clutch should be smooth and easy to press,like a honda
, even with that dual friction clutch. I have the same one on my stang. Aftermarket clutch quadrants make the pedal smooth. If the pedal is hard to push, something is up with your cable.