Have a look at this (nicked this from one of Shazaam's posts)
As you can see the basket is heavily worn, your plates will be similar also, as the plates ride up and down the basket via the clutch lever because of the worn teeth you will feel a snag in the lever as the plates catch the worn areas.
You can do one of two things here, either file the worn areas back which is the cheap option, you can only do this once or twice, or replace the basket and probably the plates also.