i'll second the "needs bleeding" notion. as the motor heats up, and the fluids get warmer in all the lines (including the clutch line from master -> slave cylinders), any air trapped in the line also heats up, and starts to expand (as gasses do under temp). the expanding air takes up more volume in the line and translates into a "mushy" feel in the lever.
bleed bleed bleed some more. if your master cylinder has a banjo, bleed there as well as at the slave and you should be good to go.