Ken,
Stability is always a problem with liner type models. Whatever you do she will always be 'twitchy' but that big gel cell is doing no good at all. Cylindrical NiMH or even Lead Acid Cyclon cells low in the hull will improve things no end if coupled with some low ballast such as the steel bars suggested. The house bricks have a high centre of gravity too so are not a good test, you need lead sheet right in the bottom of the hull for maximum effect but that may not be enough if the superstructure is too heavy.
You have a double problem, first of all the inherent stability caused by the high superstructure which you needed to have built really, really light and then there is the windage which will be considerable on a high sided ship.
Other Titanic models depend on a false keel for stability so don't rule the idea out. The thing to do is to build in a couple of mounting strong points along the keel into which you can screw alternative false keels until you get the balance right. A false keel is a small price to pay to get a realistic on the water performance, scale sail people do it all the time. Nobody is going to see it while the ship is afloat.
Don't give up yet! I am building a liner myself but it is a more 'sturdy' full bellied design which hopefully won't have the same problems.
Colin