I'll bet you just need to adjust the clutch. The adjustment is on the rod that comes out of the rear of the slave cylinder. There's an adjuster nut and a second nut that jams against the adjuster to lock it. If the jamming nut comes loose, the adjuster will gradually back off, leading to exactly the symptoms you describe.

Another possibility is that the throw-out bearing fork -- the thing the rod pushes against -- is cracking.