Update to this thread: My builder blocked and braced some of the joists. It didn't make a difference. Then, my builder hired his own engineer to do an inspection. His report stated that while the floor is to code, it's far springier than would be expected. He noted that he's 160 lbs., 5'-9" and while walking around, not stomping, not jumping, items on shelves bounced nearly off, furniture swayed, etc.
Our builder then offered to add four more beams, at a cost to us of $1500.00. My husband said; "WHAT!? Are they seriously going to ask us for more money?" We sent a message to the warranty company warning them that it was time to take legal action. The very next day our builder agreed to do the work at no extra cost AND to warranty the work.
Good news! My floor is stable. It's wonderful. All that's left now is to have the flooring material ripped out and replaced throughout the house, again. The laminate is falling apart, the vinyl has staples popping through all over. Once that is done, we can finish moving in our new home (now two years old).
I'm so relieved that this is almost over. I want to thank you all again for the great advice and information you provided. It helped me understand the problem, and make a plan to move forward.
THANK YOU SO MUCH!!
Holly