2003 ZTS - Clutch problem

Hi I have a 2003 ZTS 5speed (ZETECH engine).
My clutch gets very stiff ( like it has a super duper pressure plate) the first time or two after driving say 40 or 50 miles on the
interstate and its at least 70 degress fahrenheit ( the hotter it is the stiffer it gets). Also in extreme heat 95+ Fahrenheit it will do this even in city driving .
Obvioulsy, whatever the problem is it's temperature related.
It started exhibiting this behavior at about 28k and I now have 80K with it doing the same thing but no worse. The clutch is perfectly normal in all other respects.
Is it possible that a fluid change/flush would resolve this and if so would both the clutch and brakes have to be flushed?
Any other ideas?
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