Hard shifting after highway run

[FXDX1450]

2008 FX4 5 speed 118,000 miles . After I have been at highway speeds 70-75mph for 2 hrs steady, it is hard to down shift approaching toll boths or getting off exit ramps. After about 30 minutes of cool down or around town drive it goes back to normal. I have checked the slave fluid level its where it should be. I did have the slave replaced -it failed at 100,000 miles, I also replace the clutch at the same time (it was wore down 50%) figured I would save some time and money since it was already dropped down. I did not have this issue until after 105,000 miles.