So, one possibility here is that you are doing everything perfectly but when you hit that one section, you simply exceed the performance limitation of the equipment at the speed and rate of turn you are attempting. If this is the case you have three options. You can learn to live with a bit of chatter, you can slow down or you can decrease your rate of turn. Keep in mind that a snowboard is never going be 100%, there are subtle irregularities that will also cause it to behave slightly different on one edge or the other. These subtle differences will only be noticed at the performance threshold.
Thanks for the advice!
Update time: Went out today to the same resort I had the issue at last weekend. Conditions were very similar, temps very similar. This time I started on an easier run to get my legs into it. At a moderate speed I started doing some great heelside and toeside carves with no issues.
Brought the speed up more and no issues. Went onto a different run and there was the chatter again but only when my speed is up and the pitch steep (i.e. 30 deg or so). I lowered my speed just a TOUCH and it went away. Also I noticed on the easier runs that one my heelside if I really focused on having my knees QUITE bent (i.e. they'd be pretty close to a right angle at the joint) and pressing through the board, it was very comfortable. Got slightly lazy in my stance and back was the chatter.
So here's my lesson learned: It's a new board, it has a much longer sidecut radius than anything I've owned before. This allows the board to be on a much steeper angle given the same turning radius. This is going to have my legs at different angles than I'm used to, weight in a different location, etc. And I needed to spend time learning the actual ins and outs of the board. Today was only my 6th day on the board, and I feel like it's taken this long to start to really understand the substantial differences in it, and how to adapt my style for it!
When I went from my last board to the T7, I went from a 153 to a 159, but they were similar shaped boards. Similar sidecut, similar effective edge. This new board has a very different sidecut radius, and almost 20 cm longer effective edge despite being only 1 cm longer than my T7. It's a different animal!
One important thing I've learned is that demo'ing a board might not be the best way to test them. If I had demo'd this board for two runs, I would have walked away. After one day I really liked it (got some great toeside carves out of it on day one), after day 3 and 4 I wanted to kiss it, on day 5 I was frustrated by it (or me) and now after day 6 I can honestly say it's a better board than I am, and I'm learning new things that I haven't learned in 21 years on the snow, and it's taking my riding to the next level!