Might as well replace both of them while your down there, neither is difficult. And remove/bypass the clutch dampener with a front brake hose from a 280zx; this will make bleeding easier, and better(arguably) the clutch pedal feel.
bleed system first to make sure there is no air. this will cause a lack of pressure to return the pedal to normal. it would allow enough to engage the clutch enough to shift but not to return the pedal.
had it happen to mine when the slave cylinder was starting to go, cheak for leaks on the whole clutch system. if none then cheak the ajustment on thew clutch pedal under the dast. might have something to do with it.