1970 911T becomes hard to shift after driving one hour.

[911Tfan]

Hi,
I have a 1970 911 T that I drive often. After driving it about an hour in 85 degree heat it became harder and hard to shift like something was expanded and more force was necessary to move the shifter. Like the gates got tighter and narrower. After cooling down the problem goes away. I drove home in third so I wouldn't have to shift much and at stop lights the car stalled because the clutch was not fully disengaged even though I had the pedal down.
A while ago I had a similar problem with clutch adjustment as the clutch seemed to get further and further out of adjustment as time went on until it became undriveable.
I ended up having the clutch disc replaced as well as a bunch of the hardware. It was found the the clutch fork pivot bolt was slowly backing itself out causing the clutch to come out of adjustment over and over. I think this has been fixed because if that was the problem again it would not go away after cooling down.
Any ideas?