1998 Cavalier Clutch Problems

I have a 1998 Cavalier Z-24 i recently put a new motor in it and when i did so i put a new clutch assembly in it also even though it was not giving me problems. Since then i have had problems with the clutch. It is fine for a hour after bleeding but then becomes stiff and hard to shift. Bleed it and it is fine again. I replaced the master cylinder and it still has the same problem. Does anyone have any ideas or had similar problems?