Join groups of N lines

I have a text file generated from an old database that consists of one line per field, with no delimiter between records other than knowing how many fields there are. What I'd like to do is join the first N lines, and then the next N, and so on. Is there any way to do this within Vim? Is there were a way to select lines to apply a command to based on an arbitrary VimL expression (like line(".")%5==0) instead of just a regex?