ToME: the Tales of Maj'Eyal

Step Up gives you 1000% speed. In theory, since I'm usually at 100% speed, this means I should get ten steps unless I attack or use a skill or something, right?

Why is it that sometimes I take maybe two steps and then I lose the bonus?

What class are you? I know that, at least for Brawlers with counterattack, it's possible for an enemy to attack you while you're moving, miss, and your counterattack breaks the bonus.

Alternatively, do you have anything on autocast? That'd obviously cause troubles.

At the time, I was a Bulwark. My character is quite dead now so in order to test any theories I'd need to make a new character and level it up.

As far as I could tell, my character wasn't slowed or stunned while moving, so I don't know what could have made the buff end so quickly. I wondered if maybe Vitality kicking off might have actually ended the buff or something. I guess it's a bit moot now.

Automatic talents are a real pain with step up (or movement infusion, ligntning speed, etc). In your case, I think the problem is maybe that you block an attack and set the foe on counterstrike (which is considered as a non-movement action). I am not very familiar with bulwarks, but I noticed many other automatic actions that can break a movement infusion: couterattacks, sustained spells (like thunderstorm), psi weapons, etc...
IMHO, this can be considered as a bug. Wild speed should absolutely forbid any kind of automatic action.

You aren't breaking your Step Up through your actions.

For whatever reason some triggered action effects like this one can occur 'partway' through a round (probably due to speed modifiers on your attacks) but they still end at the exact end of the next round. As a result, you may see less than one full turn's utility from them, and it is really hard to tell how many steps you'll actually get in any given instance.

The result, in the case of Step Up, is that you'll get somewhere from 1 to 10 'steps' before the turn ends and the effect wears off. This can be pretty disconcerting when you take 5 steps into a room full of Shalore arch-mages, and you suddenly pop out of it with 3 steps left to reach your next target...

For this reason, I strongly suggest making sure you always have a backup Rush or movement infusion to close the deal.