I'm on my second C5 (traded the '09 on a '10).
Both had severe wobble over 55 mph when subjected to impulses like cross wind gust, bump in road, shake of bars, etc.
But only if I'm sitting all sloppy and loose.
If I grip the grips and squeeze the tank, no wobble. And I don't mean death grip and "contraceptive" leg squeeze, I just mean handshake grip, some knee pressure. Like how you SHOULD be attached to a little bike while going that fast.
I'm 5' 8". 195 lbs (kinda top heavy: powerlifter) and I carry my gear around in an ALICE pack giving me an even higher center of gravity which exacerbates wobble. If I ain't hanging on.
Suggest everyone do squats, benches, rows, chins, deadlifts, pushups, whatever for a few weeks. Then get back on, hang on and ride.
The preceding are the opinions of the author. Your (lack of) wobble may vary. Ride responsibly and remember to brush and floss regularly.
Later,
Mike