Cassette jerks chain forward intermittently
Since the cassette is being driven by the wheel, the freehub mechanism is catching instead of releasing.  

Confirm that by removing the rear wheel, remove the cassette, and spin the wheel/freehub off the bike (ie in your hands)

When powering it through the freehub the wheel should turn like normal, but as soon as the freehub is not driving the wheel, it should stop and will make a faint or loud clicking sound.    If you feel the freehub still being pushed around by the wheel, it needs work.

Fix0 - clean all the impacted dirt and oil, hair and grass out from behind the cassette and around the freehub.   Consider removing the dork-disk if its cracked or brittle or not sitting right.

Fix1 - blast the whole thing with cleaners, then let it dry, and lube it up again before use.

Fix2 - disassemble the freehub and attempt an interior service.  

Fix3 - replace the freehub mechanism (last resort)