clutch pedal falling

i have a 2004 nissan sentra ser spec-v 6speed, 104k miles. for the last 30k miles i have been having a prob. with my clutch. it engages fine, it doesnt slip, it shifts fine. but at times the clutch pedal will give out and i only have about half of the pedal. when this happens, there is no difference in the way the trans. acts. it still engages good, doesnt slip, and it still shifts fine, you just have to adjust for the "full engagement" point of the clutch (which is about half of what it should be). this has me baffeled. all of the fluids are full, sometimes it happens on start-ups and sometimes it dont, sometimes it happens as im driving and sometimes it dont, it happens when its cold and hot ( both trans. and weather) it just happens and i dont know. if anyone has any suggestions it would be helpful. thanks in advance.

 

thanks....