It sounds to me that you ain't getting the full movement on your clutch cable.
The clutch is automaticaly adjusted by a ratchet type system on the pedal box inside your car. I am guessing that the teeth are worn which is causing the cable to slip and gain lots of free play.
If this is the case, what you can do is pull some of the free play out of the cable at the gearbox end (not too much or your clutch will slip like mad) and hold the cable with some vice-grips against the release bearing lever.
I hope I have worded that correctly and that it is useful, only for limp home mode though or your grips will go rusty and it's dodgy.