from your excellent symptom diagnosis, excess heat is causing the problem. Now--insulate the slave cylinder and lines. Either use industrial white woven insulation tape or some kind of flexible tubing over the lines.

change the fluid to ATE Blue (higher boiling point). Remove the radiator and have it 'rodded out' to remove the mineral scale that's causing your overheating engine problems (or replace the radiator-- aftermarket one from radiators.com)