Programming :: Sed - Replace Alternate Occurrences (on Different Lines)

I'm trying to process a postscript file and I want to change alternate pages by finding the string '%%EndPageSetup' (and then adding a line after it).

I've tried using sed with labels and a branching program to ignore the odd occurrences and process the even ones. In the code below I tried searching for the first and ignoring it by jumping to next and processing it and then to the next odd page and so on.

My logic is obviously flawed here as it doesn't work but I've tried the following:

Code:
#!/bin/bash
sed -e '/%%EndPageSetup/{
b even
:odd
/%%EndPageSetup/ b even
:even
s/%%EndPageSetup/%%EndPageSetup++/ b odd}' filename.ps > newfile.ps