what year is that 5.2? If it's pre-1984 then there's a good chance that it is NOT a foam core boat but is solid fiberglass. There is a tube stringer that runs the length of each side of each hull and it may have either separated from the hull or degraded completely.

Whether it's foam core or not, you probably need to remove the deck to get a really good understanding of what's happening. If you can reach inside with a digital camera, it might also enlighten you a bit as to the problem.

Here's some inside shots I took of the Team Seacats I20 as I was inspecting all the structural areas before a Tybee 500. It was impossible to put my eyes on these areas but the camera extended through a port hole worked wonderfully.