I know this is a pretty old post but my X has started doing exactly what you describe. Worst in 1st and 2nd gear. Sounds like birds. My X only has about 60k miles or less but has been through some pretty rough times over the years and terrain across the U.S. I am not familiar at all about clutch work or harmonic balancer work at all. What are some things to do to prove one thing or another, as I would hate to dig into one thing only to find out it was the other? How long can you drive like this before failure? Mine has only been doing this for about 2-3 weeks (that I have noticed) but does seem to be getting worse.