I'd be worried about the flexing. It it's an ES-335 it should have a center block so it would not flex any more than an SG or LP...which is to say, not much. Gibson ES-330's did not have center blocks. My experience hollow thinlines is that they are pretty sensitive to temperature and moisture, and go out of tune easily, but they should not have more flex than any other acoustic guitar.

So if there is much flex, there may be some breakage or cracking internally? If the joint has not come a bit loose, then my next guess would be the wood is somehow loosing it's integrity. If that's the case, it might be less work to replace then neck than to fix it.