Your FPS has a direct impact on what happens when you levitate. Tab out to a window that partially obscures your eq window, and watch yourself rapidly descend. Conversely, I can minimize EQ for a span of time, and actually levitate upward. I used this trick to get on top of the Temple of Marr in North Freeport to demonstrate, since there's no other way to reach this point as a player. (There's a rooftop about the same height but you'd sink before reaching the temple)
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In ideal hovering conditions, you will move up and downward along the z-axis at the same rate, unless you are high enough to sink due to game mechanics (too far above the ground to hover). So, it's more of a client issue than anything, but knowing how it works (WinEQ2 and EQplaynice artificially lower your FPS) may help you prevent the problem.