clutch building pressure and staying engaged?

I have problem with my clutch on my 1990 90 (supposed to be a turboD but its now a 200) where the pressure is building in the clutch pedal. one minute its fine, if a little heavy, then all of a sudden when you push it down say at traffic lights it stays disengaged, the pedal returns up fine but the clutch is still disengaged. If you then pump the pedal to see if it returns it just gets harder and harder until it feels like the brake pedal. I have changed the master and slave cylinder with no change, and checked the pipe which is fine.
Now if you leave it over night and you can drive it again the next day for a few minutes then it happens again! any ideas?
Thanks!