Odd... Quite possibly the clutch. Might as well try a new cable(FMS adjustable if you got the money). And if that aint it, you could use the cable anyway. Anything other than the cable or friction in the pedal, is going to require a tear down. And while your there just replace everything to save you from doing it again in the near future. Clutch, flywheel (or resurface), replace fork pivot if its scarred from wear, new TOB, pilot bearing.
By your description, its sounds like metal heating up, and clearances shrinking. But I cant think of anything that would be affected by that.
By your description, its sounds like metal heating up, and clearances shrinking. But I cant think of anything that would be affected by that.