Has this been getting worse over time?
My guess would be that if there is no binding in the linkage, that it's either the pressure plate has some broken springs in it or the release bearing is seized and wearing through the collar that it is pressed on to.
I would disconnect the adjustment link that goes through the fork and move the pedal back and forth to check for binding or loose points in the linkage, and if not, it's internal. Remember, the over center spring is strong! (it might be a good idea to remove it to be sure) Maybe watch the linkage movement with everything hooked up and the engine stopped to see visually if there is any deflection in the linkage or unnatural movement or noises. Start the car up and see if anything changes doing the same test.
I'm taking for granted of course that your engine and trans mounts, bushings, ball studs, bell crank, etc. are all correct and in good shape. If you have a really stiff pressure plate, there is a chance that your firewall might be cracking too, but it isn't all that common.
And yes, (believe it or not) i have run across all of this before!