1998 Subaru Impreza Clutch pedal loses travel

Only in hot weather, and only in stop-and-go traffic or around town (with frequent clutch use), the clutch pedal travel gradually increases until it's on the floor and changing gears is impossible. Clutch fluid was changed approx. 175,000 mi., no help. No leaks, fluid remains full. Fine in winter, regardless of amt. of clutch use! Appreciate any help -thank you!