stiff clutch

[giorgos anagnostakis]

greetings from Greece! I have bought my car at 2017. From the beginning and all these years i have a problem that the local nissan cannot fix. even though they have changed eveything again and again. 
When i travel on the high way for a long time, when i press the clutch (from 6th to 5th ) it feels "heavy". after i press it from 5th to 4th it gets normal again. (there is no problem with the gears)

when i drive in the city or small distances that requires pressing the clutch often everythings works fine. The only problem is on the long distances when the time comes to press the clutch its seems to loose the hydraulic pressure. It gets really problematic when you travel for 5 hours or so..
Is there noticed the same thing? 
I have changed all these years almost everything that relates with it. It seems that the fluids are loosing their power from heat. (heat from engine?)