Why does my hoverpad flip over?

I tried building a "pad" that would allow my Kerbals to fly around at moderate altitudes, hovering in the air; something like a quadcopter, just with jet engines.

I made sure the center of thrust is above the center of mass, that way the thing should be self-stabilizing. Well, it isn't. It flips over a few seconds after take-off.

I thought the center of mass may be still too high. I strapped the huge Roxkomax X200-64 fuel tank on the bottom to weigh it down, and still - it managed to take off despite the very heavy load, and promptly flipped over.

What should I do for a hovering contraption so that it can hover unattended without flipping over?