I don't think the Infinity is stiffer than the Lunatique, so the chatter is probably a technique/weighting issue. Which is pretty common for beginners progressing to intermediate. I suspect you're talking about that heelside turn chatter/breaking lose. If so, that's not a limitation of the board.
There's a different kind of chatter that happens when you bomb a run on a short/soft board, but I'd be surprised if this is what you are experiencing.
Buying a new board is always fun but I don't think you've "outgrown" that one based on your description.
Are you comfortable on black runs yet? If not, you'll wish you had a shorter board when attempting them the first time. 146 is a pretty reasonable size according to the weight chart of that model (note that board length does vary depending on model, 146 are not all alike). If you try steeper/faster stuff before you really have short radius dynamic turns dialed in, and your board is too long/stiff, you're going to find that about the halfway point across the fall line, the board will lock in and take you for a ride.
If you are bombing down the hill very fast, and the tips of your board are vibrating, and this is what you are talking about...then yeah, you progressed very quickly and you probably want a new board.
Last edited by jtg; 03-23-2014 at 11:10 PM.