Slave cylinder ?

[Please help]

Hi all .....this is the first time I've ever posted on here so I apologise in advance if I'm posting in the wrong place !
I've got a 2011 transit van ef11kus 2.2 diesel 122k when bought got 134 k on the clock now ....so here's my problem I normally Day to day don't do any mileage but every couple of weeks I do a 350 mile journey on the motorway and this is when my problem starts after being in 5th gear for like 3 hrs going 80 mph when I come of the motorway I've no clutch can't get into gear ,hazards on pump the clutch for a while sort of come back enough to engage and get me home ,then driving around town it seems to heal itself ......so I've had the system bled didn't sort it on my next motorway journey ....so I had a clutch master cylinder fitted didn't sort it on my next motorway journey ......so is it the slave cylinder next ? .....if so why does it heal itself around town driving ....oh and I'm not leaking fluid either......I just don't want to be spending the £ if I'm barking up the wrong tree ! .....please help folks !