On behalf of a friend, what would cause your gear changes to become difficult once the car warms up. While his car is cold, it shifts perfectly, but once warm he finds it difficult to engage gears.
Rubber piece of Clutch line between clutch slave cylinder and master cylinder.
When it is hot causes the rubber piece to expand (lose shape) and lose some pressure. Replace with a steel braided line or have a complete metal line made up.
