[Shotimes] Suspension? Advice

[Beisler Jonathon]

Greetings,

I have a 94 ATX.  When I bought it, about two years ago, all four of the
tires were cupped and my steering wheel was dancing in my hands constantly
so I had to replace them.  I was told this was because the struts had gone
bad.  I heard this from both the tire place that I have been going to for
years and several different shops.  So, I replaced the struts and then
replaced the tires and everything was fine for about a year and a half  and
20k miles.  Now, gradually, the whopple in the wheel has returned, but it is
only between about 40 and 55 miles per hour.  When I am cruising on the
highway at anywhere from 75 to 100 the ride is smooth as silk.  I have had
the tires rebalanced twice and the problem still persists although it seems
to be better for a little while after the balance.  One tire guy tried to
tell me that my tires had flat spots, probably because of the way that I
drive.  But the manager with whom I have shod many a car with many a tire
said that was not the case.  So, if the tires are cupped, what can be the
culprit?  You could actually feel the cupping as well as visually see it on
the original tires, but the newer ones feel flat and even.  Can this be an
illusion created by warped rotors?  I don't know anything about suspensions,
really.  
If any of you all have any ideas about this, I would greatly appreciate it.
Thanks,
JB