97 t-bird lower ball joint keeps popping out. Why?

I have a 97 thunderbird and I keep having the lower ball joint pop out on the
passenger side. I pressed the ball joint out once and pressed the new one in, it
seated just fine. Drove it to the store and when I backed up it popped out.
Thought that maybe I didn't get it pressed all the way. Took it to a shop and had
them press it out and replace it. Drove it home and turning into the driveway it
popped out. Figured the lower control arm was bent or out of tolerance, so I
replaced the control arm, the new one had the ball joint already inserted. Put it
on and was driving down the road at highway speed when it popped out again,
burying the wheel assembly back into the wheel well. Needless to say that I was
incredibly lucky that I wasn't killed. Why is this happening? The car has never
been in a wreck and is very well maintained. One thing I noticed was it looked
like the strut tower is separating from the body. Each time the alignment was
corrected before I drove it. I'm at a loss on what to do, except junk it if I can't
find an answer. Any ideas?