A plugged cat would make a chugging like that, but what both Steel286 and I both noticed is that an engine restart "fixes it". - Seriously - car acting all jittery and wierd at all RPM's - kick it into neutral while still cruising, restart engine, kick back to drive, and the problem is gone. I don't see that being cat in my case. Also, at least 90% of the time I drive it, it runs perfect. Gotta be a messed up sensor.