As I get more miles on the Y, the "jittery" feeling going down the road is getting a bit better.
Keeping a light touch on the bars and focusing your attention FARTHER down the road helps.

BUT.....I decided to put a bit more pressure in the tires to see what that would do.
To my surprise, as I added 5 PSI over the recommendation to each tire, that caused the axle/frame to raise up noticeably; maybe a half to 3/4 inch.
And it seems to have made the ride more stable too (sidewalls flex less ??).

Some additional seat time is needed to see for sure but at the moment I'm happy.
I suspect that tires with stiffer sidewalls might be even better.