I'd carefully check the hose running between the firewall mounted reservoir and the clutch slave looking for any heat sources nearby that might be causing excessive heat. If you have an under car exhaust as opposed to sidepipes I would also check nearby the exhaust manifold and pipe running down from it as a possible heat source causing things to cook. I think the basic problem may be some sort o heat source too close to the clutch reservoir hose or to close to the clutch slave mounted near by or on the bell housing.