Don't worry about teasin'. I've been called a heck of a lot worse than a Baldwin. Many of them unable to get around the forum
The PR going down one per click is fairly normal. It naturally happens because of the nature of PR. The deeper the page, the less likely there is to be a direct link from an outside site, so you're relying on your own internal linking structure to pass PR. And, try as we might, we can never pass more PR than we have.
Another factor is that it takes a PR update for PR to be calculated. Often a page is found, makes it into the index during one update (yes, it's a little different nowadays, but...) and a "guessed" PR will appear for another month before real PR gets applied to the page. It takes an extra month for real PR to be calculted for the page. On dynamic sites, I've seen it take as long as 6 months for a page to get real PR - mainly because it can sometimes take Google that long to identify what it is that makes the page a unique call, I suppose.
At the end of the day, though, it reaches a point where you just need to kinda start treating PR for what it is and not take it too seriously. You need a good base PR and you need good inbound links. It's something you should always be working on, but when it comes down to it, PR doesn't do a heck of a lot for ranking pages - espcially deep ones. At a certain point for "specific terms" (which your deeper pages should be) you can blow a PR6 or 7 page out of the ocean with a PR3 page. This is where your link structure and on-page elements come in.
The trick here is that we don't know what's on each of the various archive pages you've got until we look at them. So, it'll be pretty hard to telegraph anything. The pagination of the various directories as I suggested in the first post may work, but you're not going to get any useful inbound link text in there. I can't (with the limited knowledge I have on your site) imagine a programatic way of handling this optimally. You can whip something up and it'll get in the index and call it a day. Or you can hand link to things optimally but you'll be at it for a decade and by then, the linking rules will have changed.
If your site is fairly active, maybe you can generate some interest in getting others to help you out. Send the teeming millions in there and get them to start linking to their favorite ones. If it's like most forums, maybe 10-15% of the posts are really worth indexing for posterity. Discussions and debates are fun while they are still active, but have little value once they'd died out. The constant repetition of the same old questions (such as here and the "Help! My Site Is Gone!" posts) is natural because it's hard to organize a forum so that it's easy to find things. There's nothing wrong with them, but there's no particular use in reindexing them.
<shrug> It's a tough call because there is no right answer. There are plusses and minuses to each option.
Not an answer, I know, but hopefully it's useful in some way. And now, I'm off to shoot a movie.