Interesting, you still on the stock clutch? I get a similar feeling on occasion with the Exedy + Fidanza combo, but I know that's just down to the nature of it at low speed and me being slightly useless: It doesn't do it when I'm at speed with high tempo shifts. Could well be flywheel related, or a combination of that along with an adjustment needed on the clutch pedal itself? Might also be a slave or master cylinder issue, dodgy seal and it's jumping when you don't expect it.
Does it do it when hot, cold, or random?