i had a similar problem like this once before. not saying this is your problem but, the shifter lever that is in the case that has the spring on it has a rod that it pushes on when you pull the clutch. on the other end of the rod inside the case is a very small ball (like a ball bearing) in between the two push rods inside the clutch. i'm not sure why but that ball fused itself to one of the push rods and the tention in my clutch was completely gone. cable was fine and clutch was fine. i replaced the lever that goes into the case, the two push rods and the ball and problem was solved and has been fine ever since. again im not saying this is your problem, but its something too look at.