Help me figure out what the hell is wrong with my vehicle

[Ziff]

Uh... No. Your center of mass moves backwards as you burn fuel. You drain from the top tanks first, so the top end of the rocket gets lighter. This keeps moving the center of mass back until the last tanks drain, then it moves forward again.

Except there are two problems. He has 8 NERVA engines on top along with 2 large fuel tanks that do not drain downwards. So fuel isnt actually draining out of the top of that craft, its kind of starting in the middle. Because of these two facts the center of mass doesn't actually move down as it would in a craft with a lighter top. Instead it moves down only very slightly before it starts to move up because the top is so darn ridiculously heavy.

Top-heavy rockets are more controllable by thrust-vectored engines for the same reason that it's easier to balance a broom on your finger from the handle end than the bristle end. You have a longer lever on which to act.

This is simply untrue. The farther away the rocket engine is from the center of mass, the harder it is to control. It will actually act like a lever and become uncontrollable. In his case he has a very long, very top heavy rocket. Because it's so top heavy, and because the fuel at the top isn't being burned off (it's starting in the middle) his center of mass moves upwards far sooner than a typical rocket would see. So it very quickly becomes uncontrollable. This is also another reason why rockets stage. Shorter lever, more control. Longer lever, more action (uncontrollable).