The problem might be caused from the clutch master cylinder being effected by heat.
If the clutch master cylinder cup seals are at all worn, as the master cylinder gets hot from the engine heat it causes the bore inside the master cylinder to expand slightly. The worn cup seals can no longer seal inside the bore and the proper hydraulic pressure isn’t produced to fully disengage the clutch.
You can confirm if this the problem by pouring cold water over the clutch master cylinder when the shifting becomes difficult.
I’ve had to replace two clutch master cylinders in my vehicles because they were effected by heat, making shifting difficult if not impossible.