There can be many reasons for things like this.. But sometimes what you describe can happen if the release bearings internal bearings are worn or slightly oval, especially if you're stamping on the clutch pedal trying to change gear very quickly. What happens is the front face of the release bearing doesn't rotate smoothly like it's supposed to when it hits the clutches cover springs/fingers, so the bearing face actually skids slightly around the clutch cover springs giving a grindy type feel through the pedal.
Also if the spigot bearing is loose or worn, at high RPM's or certain frequencies the clutch pressure plate can vibrate on the input shaft knocking either the clutches machined face or flywheel, or both giving a weird pedal feel.
Engine harmonics can also effect the feel of the clutch pedal at specific engine/clutch/gearbox RPM's causing vibration or changes in pedal feel, which at any other RPM wouldn't be noticed. Sometimes dampers are fitted in the clutch operating mechanisms to help cure these issues.
If the problem persists though It should be checked out further.