Yep as 6pk2goDemon said, the whiskers make a big difference.
Worn rollers can add to it by letting the bottom of the glass move around.
One thing I would mention also, is that my glass kind of slopped around a little when they were down and they rattled on bumps and such.
A slight crimp toward the bottoms of the vertical tracks to keep the glass from getting down below the whiskers helped that a ton in stabilizing that.
Ok next.
Under that trim at the top of the door panels is an adjustable piece that used to have felt on them, and those are to help stabilize the glass not only when up or down but in all positions.
Make sure the felt is there and that those are adjusted up against the glass.
If the felt is gone go buy some from an upholstery shop and contact cement them on the metal tabs.