The car is a 1991 Mazda Miata with 180,000 Km. These cars have hydraulic clutches.
After engaging and disengaging the clutch repeatedly, the catch point becomes progressively lower until the car starts to creep forward with the clutch pedal fully depressed.
If I pump the clutch repeatedly 5 or 6 times, the catch point returns to a more normal position, and the car no longer creeps forward with the pedal all the way down.
The fluid reservoir is full.
I suspect that the problem is air in the hydraulics lines, but could it be something else?