Robbie-T

Hi all I'm near to this forum, I live in Australia Melbourne. I have a black c3 corvette 1980 manual, I love the car even it's faults.
My problem is the cutch pedal, when it is cold it shifts fine and is easy to use. Once the car has been driven for awhile the clutch feel heavy and hard to push and it's sometimes hard to shift into gear.

Any suggestions as to what this could be