had the same symptom a while back after overhauling the master and slave. first off make sure you ain't leaking fluid. if all's well i'm thinking you've got an air bubble, which could mean a loose bolt up high in the line (e.g. at the master junction). another thought--do you have stock lines? if so, there is an intermediate connection that sits over the middle gear cover. i remember mine being ungodly torqued shut, so odds are there might not be a prob there on yours, but there's always an exception. good luck, prob an air bubble finally loosed itself from some crevice--bleed the line
what really helped me, though, was my shopvac bleeder. i couldn't get my brake lines to burp up a bubble. using the vaccum bleeder fixed it up good. anyways, i bled the clutch lines with it and it firmed up the lever action even more then i ever could when i used the conventional method.