Fussiness under the baseline


I'm cant find a solution to this little problem:

If you look closely, will see that at 14 and 16pts there seems to be grayscale pixels under the /n serifs. How can I remove it, so it looks like the ones at 15, 17 and 18pts? (Firefox/Mac)

The blue lines are already in place, like this:

However, the problem remains.
I spend all day experimenting, hinting, un-hinting, tweaking blue values, looking at pixels, etc... and wasn't able to find a solution. Nothing seems to make any difference.

The greyscale pixels below the /o are OK, but not the ones below the /n, /i, /l, etc...