Something I completely forgot to mention IS the clutch linkage itself, from top to bottom. Examine each section carefully, you'll need a second pair of eyes. While slowly working the pedal up and down, look for bending or movement on the various links.
The frame mount could be loose/ rusted/ bent, the z bar setup could be cracked and moving under load. On some designs the fork itself can be cracked, say, on "one side" of the stamping and bend just a little under load
