Drawing a String looks ugly when the text is moving

Question

I'm drawing a name above the player's head in a 2D top down view game. The code to draw the string looks like this:

Vector2 textPos = new Vector2(
  (positionX * 2 * world.Map.TileWidth + (16 -(SmallFont.MeasureString(mName).X / 2))),
  (positionY * 2 * world.Map.TileWidth - 32));

spriteBatch.DrawString(SmallFont, mName, textPos, Color.White, 0, Vector2.Zero,
  1, SpriteEffects.None, 1);

The text is drawn to the position right above the player's head just fine, and it follows the player as he moves just fine as well. But the problem is that when the player moves, the text kind of gets thicker and ugly. I tried screen capping it but the screenshot doesn't capture the problem (because it's just taking a single frame, I presume).

Anyways, does anyone have an idea of what I could do to fix this?