Erratic froth - does anybody know why?

[Maus]

Since getting our espresso machine about 3 years ago, we've become pretty adept at floating the steam wand at the level of the (usually 1%) milk to get a nice rich froth. Unfortunately, this works only about half of the time. The rest of the time, we get froth ranging from fair to poor (just a lot of big bubbles). Sometime, with the same carton of milk, we can just not get decent froth for a day or two, and suddenly we get nice froth.

Does anyone understand why this is happening, and what we can do to get good froth more consistently.

Thanks,

Maus