Clutch issue on 9-3 TiD manual gear

Hi there.
While driving for more an hour without touchinh the "clutch pedal",
the pedal becomes very soft while pushing it and stay sticked at the
botton and have no effect in declutshing. I have to pull it up
manually and pump a dozon of time in order it gets a normal hardness
and plain de/clutching role !
The softness of the pedal is linked to the duration I not using it.
This problem never happend while the first start of the day, and never
happens while driving in city, only on the highway.

Any idea... trick... recommandation

Thanks for your colaboration