It could be a number of issues. The best bet would be to start with basic maintenance.
Remove the slave cylinder while leaving the line attached, and remove the operating rod. Clean them both, inspect the O rings on the rod for damage or swelling, apply a light coating of grease to the rod ends and O rings, then reinstall the components. Follow this with a proper bleeding of the clutch circuit.
Degraded fluid, worn seals, and sludging can all cause symptoms that are exacerbated by heat.
Additionally, ensure that your levers are properly adjusted with some freeplay at the end of the movement. This allows the components to heat and expand in the engine case, without resulting in a partially engaged slave cylinder causing the clutch to lightly slip and overheat.
I have also had issues with issues with the master seals on bikes that were poorly maintained before I purchased them.
Typical slave failures result in leakage, frozen pistons, and notchy operation from scored bore walls.
Typical master failures involve leakage, drag during operation or release, and internal leakage from damaged seals causing clutch drag that typically results in shift failures such as grinding, inability to engage neutral, or engine drag while lever is engaged.
If it is a loss of pressure from the slave, the leakage will have to be external and visible. If it is the master that loses pressure, the leakage may be external, but will more likely be internal and can push back into the reservoir.
