What is under your TT, cheap islolation?

You have two other solutions make the floor stop bouncing or get a turntable that is immune.

a. You seem to shy away from any type of structual reinforcemnt to your house. Heavily padded carpet might absorb some of your footfalls.
b. Most turntables today have no suspension at all. Those that do are not designed to deal with the magnitude of oscillations you decribe.
c. You need a turntable that floats the arm and table together. Even though your floor is bouncing the arm and platter "float" together and the cartridge remains in the groove. Sota and Linn make such tables.

Ultimately a major floor bounce is really an unacceptable situation for a turtable. For example a cd player has error correction and will continue to play even when it is mistracking. A turntable has no such correction. Even though it is still playing, it is still mistracking.

The only real solution is to stop the floor from bouncing or a wall mount.

I don't really know how to support the floor better. My room is located over my garage & I use the garage for storage of vehicles. I couldn't put a bunch of support collumns under the floor because it wouldn't allow me to park any vehicls inside.

At this point I would rather not put up a wall shelf but it seems to be my only option.

I am pleased I made the sand box, it was fun & came out great. I plan on using it under the turntable regardless so it wasn't a waste.

Fishwater, I have a listening room over the garage and have put in two simple ways to reduce the amplitude of floor vibrations. First, lag bolt 2X6s across the joists down the middle of the floor. Second, put a single post in the middle of the floor in between the two cars. If you put it forward of the front doors, it will not interfere with getting in and out. This combination will significantly raise any floor resonances and their magnitude.

Perhaps rather than hanging a shelf on the wall you could just attach the existing rack to the wall so that it cannot rock when the floor flexes. In my case, the motion that caused the problem seemed to be horizontal, not vertical.

As an update, I cut some raquet balls in half & installed them under the isolation platform. It certainly helped with the footfall issues but if the TT does get moving the balls keep it moving, the needle bounced almost half way across the record until it stopped. Under normal conditions of moving around in the room the raquet balls are a definite step in the right direction.