Same issue, here, been chasing it down for a while, no luck.
Happened much more in the winter than the summer.
I had the same thing with WOT then pulsating as previous post, then I had new clutch+PP put in. Now it only happens at high idle/1500RPM, never after WOT any more. Only happens while the pedal is moving as other posters have said.
The answer is probably that one of the pieces being pushed/moving, probably fork on the bearing or bearing on PP, is vibrating side to side, but not in and out, due to drivetrain vibration. Given the engine pistons push side to side, it makes sense the vibration is towards the tires, not towards the driver like the clutch fork/pistor.
Only when the piece is travelling and pedal is going in and out, the vibration is felt due to it's not so smooth travel due to the lateral vibration
Question is, why this much driveline vibration with the clutch pressed in, and why does it vary.
I tried changing the master cylinder, no help.
Somebody told me they added fender braces and front underbody braces and it has helped to lessen the vibration.