It's P2P, so there is no server, but could there could be something similar to trackers for bittorent? A tracker is not a server made for hosting anything but it's a server assisting the transfers. I'm not specialist, but I think on PS3, PSN could be acting as a tracker.
When I used Bittorent (a long time ago), I often had issues like slow downloads, or difficulties to connect to peers. While I knew for sure there were a lot of peers, I could only connect to 1 to 5 max.
Those were almost allways rooter issues, and forwarding the ports was often the solution.
There were some cases though the problem came from the tracker. In that case, there is nothing you can do realy.
And rare other cases, it could be your Internet access provider, are you using something special?
In France, P2P like bittorent are severly monitored because of illegal download of movie and music and there was a question some years ago about simply completely blocking certain P2P protocols (I don't know how it's even possible). Fortunetly it didn't happen, but the surveillance is stronger than ever.
You said you forwarded the ports and that your PS3 is in DMZ. I think it's useless to do both at the same time. If your PS3 is in DMZ, it's like the transfers are made without going through the firewall, forwarding ports must be useless then.
I wonder if this kind of double configuration could be sometimes causing troubles.
I could be mistaking as I'm not a specialist at all in the subjects, those are things I read when I was trying to find solutions for my P2P issues.
For me, in Dks, it's the opposite. The online features didn't work very well, I managed to get some PvP/co-op action, but it was rare, and my everyday was "summonig failed" and "failed to invade".
Then 2-3 month ago it started working better without me doing anything new... I even wondered weither the last patch actualy corrected something... But seeing that other people still have problems, I guess not.