I moved into a 25 year house (built in 1979) last summer which has
hardwood floors throughout. While I love the hardwood floors I have a
problem in several areas on the first floor with "floor shake".

On several spots, If I walk across the floor, there is excessive
"shake" on other parts of the floor. For example, I'll walk across my
dining room and the china cabinet which is placed againced the wall
will will move (presumably up and down), enough to make noise. Same
with the TV cabinet in the Family Room. But I can't ever hear the
floor squeak.

My guess is that the subfoor in certain areas has pulled away from the
joists causing this problem. Which is the same problem as squeaky
floors, correct? Is this just an extreme example of squeaky floors
without the noise?

This is a 1st floor problem only (why?) and the joists + subfloor is
exposed. What should I be looking for to identify the problem from the
basement? How do I fix it? Shim it or tighten it up with screws and
such? There seems to be a number of solutions for squeaky floors these
days... what's the best solution provided that it is indeed squeaky
floors that I have? Thanks!