help with squeaky steel joists

I have a house built in the 1970s that has steel joists on the main floor, with 3/4" T&G plywood subfloor. The ply was screwed to the joists with self-tapping metal screws, but also nailed (possibly a later "fix" for squeaks?). The floor squeaks and is not solid - you can see it move when walked on. I have taken up the screws on a sheet of ply (many of them just spun freely) and removed the nails that weren't sunk below the surface. I replace them with a thicker screw (not self-tapping). I still have squeaks. My neighbor said that he just put down a second layer of 3/4 ply, and that got rid of almost all squeaks. Is this my best option? Glen