This has been posted about before. I have, in fact, read over several older threads on the topic. All of them get to the point where they say basically, "set $cat and $post_per_page at the start after $blog=1; and you should be fine" but the people that posted said, "but I'm not."
I'm still not either. The getting it to load one category on a page is no problem, but $posts_per_page doesn't seem to work. Can somebody please give me a definitive answer on this? Thanks.