Check that the inner AND outer clutch cable is fitted securely, and not coming out of its specified locating position when the clutch pedal is released, as this would give the effect your experiencing as the pedal would effectively gain extra pulling travel on further presses once the cable had become dislodged. It's similar to severly over-adjusting the cable. A bit like like putting washers behind one of the cable ends, thus pulling the other end of the cable much further effectivly bottoming out the clutch cover fork springs giving a hard pedal around 3/4 travel.
If the cable mechanism is ok & working properly then its likely time to pull the transmission. Check that all the pressure plate springs are where they should be, I've seen them come out before and find themselves a new home between the cover and plate. This could definately create the pedal travel issue if a dislodged pressure plate spring is bouncing around between the cover and plate. Centrifugal force would likely keep it out of the way most of the time, but it could certainly move around on initial engine start or shut off giving and occasional issue.
Check also that the clutch fork is operating properly & that the release bearing isn't jamming against its pivot or along the input shaft sleeve. Also make sure the release bearing isn't coming off its retaining clips and skewing during travel possibly pushing the clutch forks in further etc.
