I personally haven't tried the above suggested fix, but I did try something else that has had some success (I say some because I've only tested it in a limited manner, so to speak). I'm not sure if it's necessary to do it after each wave (probably would be to have the best results), but after remembering a suggestion someone else in another thread said to try (can't remember which thread), I decided to do a little test.
Originally, my test was going to see if a vanilla System folder was going to work. For this, I verified using the Atlanta content server. No dice. So I tried the files linked in another one of the threads. I don't hang at the connecting screen much (haven't tested it much), but still no dice.
Then, today, I decided to mess with my cache settings. I set it to the recommended amount (in my case, the maximum recommended amount since I have 4GB of memory). That didn't work. Of course, I had a bit of fun testing it anyway since I turned on cheats and played some solo to pass the time (with enough practice, it could be a viable way to attempt to invent strategies for certain maps or weapon usage; needs more testing, though).
Then, I remembered about another thread I read the other day. Something about using "obj garbage" after visiting the trader or something. I decided to use that in tandem with the "flush" command after every wave (though after a point, I stopped visiting the trader because, like I mentioned above, I had cheats on to make it a little easier to pass the time), and after I took down the Patriarch, lo and behold, the next map loaded like a charm. I have not disabled my pagefile or any other possible fixes that I didn't already mention before this worked.
This requires some more testing, of course, but I think I might be on to something.
On an unrelated note, on my first initial test today (setting cachesize and changing one precache setting), when I made it to the Patriarch (BioticsLab), I went into the U-room and welded the door next to the trader, waiting around a corner on the other side. The Patriarch spawned, and he appeared to be on the floor above, next to the other trader. To my surprise, however, no more than three to five seconds later, despite not even moving in the right direction to reach my location, I hear something banging on my welded door. I poke my head outside, and sure enough, there he was, banging on my door with rockets. Now, mind you, this is the first time I've seen this happen, but it was pretty screwy nonetheless. I just thought I'd mention it in case it gets to be a problem sometime later or maybe to try to get a few laughs out of it.
EDIT 1: Just ran another test (BioticsLab through Manor (was too tired to play Manor though)) with some help from my brother (console cheats are disabled in multi-player; bummer). For best results, doing a flush/obj garbage combo between each wave is recommended. The longer you wait, the longer obj garbage takes (haven't tried either by themselves yet; trying that tomorrow). This is especially important if you don't want to lag out or are hosting a listen server. Anyway, it's working for both myself and my brother, so I think I'm on to something here.
EDIT 2: Getting ready for my second batch of tests. Thinking about making my own thread with my findings, though. We'll see once I get started.