You're not making this easy for us, yah know?
In your three or four adjustments in the past year, do you keep repeating the same adjustment on the end stub of the cable (at the transmission)? In other words, does it look like the nut(s) is somehow slipping back on the end stud of the cable? Repeating that 3 or 4 times should mean that you're getting close to the cable end of the stud on the back end of the cable. If that's true, then one of four things can be happening:
- The cable is stretching (but I've never seen/heard one stretching that much)
- The adjustment wingnut is slipping or backing off (You've checked that)
- The Bowden Tube is somehow messed up and decreasing the bend, allowing the cable to "stretch" by decreasing the distance from pedal to transmission arm (very rare, but possible and this is my current "most likely" failure) or,
- The tube inside of the tunnel that guides the clutch cable is loosening up from it's welds and making the clutch cable adjustment meaningless. If THAT is the case, you should be able to see the tube flopping around at the rear where it exits the frame ahead of the transaxle - It should be welded fast to the frame and not flopping around and can be easily re-welded by a competent welder with the car up on a lift.
If it's at the front, you'll need to remove the pedal cluster and reach inside of the tunnel to see if it's loose there. To fix THAT you'll have to cut a hole in the passenger side of the tunnel for access, re-weld the tube end to it's hanger and then (per Stan and Danny's suggestions) weld that removed panel back in place on the tunnel. It's a big job.