Firestone tires bouncing

[Pete Amerson Jr]

Back in Feb I had all tires replaced with Firestone. They put a dust
type material in the tires to balance them while it is being driven. I
then drove it about 10 miles to storage. A couple of weeks ago I drove
the RV and when I got to about 75 mph it started bouncing on one of the
back tires. I carried it back to the tire store and they checked the
balance and said that it may be the tire or it may be the rim but they
would put another tire on anyway. Now it has a bounce about 78 mph.
Is this to be expected with firestone tires or should I not be driving
that fast anyway?
Since I got the tires I have put anout 200 miles on them, all below 70
mph.

Pete Amerson
87 PT-40
Atmore, AL