First and last classes attributes problem with grouping fields

Hey Guys,
this is the situation:
I have a 12columns grid.
I want to display items 3 columns wide each on a single row(Striping class attributes:grid-3).
So a maximum of four element per row(FIRST/LAST every nth :4).
With FIRST class attribute set to alpha and LAST class attribute set to omega

Yes 4x3=12

Ex with 3 items:
Row1: item1(grid-3 alpha) -- item2(grid-3) -- item3(grid-3)

Other Ex with 4 items:
Row1: item1(grid-3 alpha) -- item2(grid-3) -- item3(grid-3) -- item4(grid-3 omega)

So far so good this always works perfect.
The first item always has a class of alpha and the last item a class of omega this always fits perfect in a 12 columns grid.

Now!!! if you wanna do the same thing with a grouping field this is what you get:

Ex:
Row1: item1(grid-3 alpha) -- item2(grid-3) -- item3(grid-3)

Row2: item1(grid-3 omega) -- item2(grid-3 alpha)

Because FIRST/LAST is set to 4, moving to the second row doesn't give the first item an alpha class if there isn't 4 items in the previous row, which of course screws up the grid now.

I hope I made myself clear enough! this bugged me for few hours nows!
Any Help would be appreciated.
Thanks a lot guys

Ace