My friend can't join my party or I can't join his party. Both have party privacy set to open and it works really randomly every now and then. Anyone know how to fix this? PS3 if it matters.

Count your blessings

I also have not found a solution to this problem. I have one friend that I'm unable to game with unless we use a third friend to host. We both have open nat and open privacy. Tried resetting modem, router and xbox with no results.

I had this same problem with a friend and once he got a new router and bitched out his provider some the problem was fixed.

reset you game on both ends?