Crazy online problem with B:P

Hello, at start I'm sorry if I put this topic in wrong place :) but I got this crazy problem while playing online in Burnout Paradise ! Ok straight to the point - every time when I join host with more than 4 player I got error saying that my system lost conection to the servers,  it also kicks me out of PSN!  But if I join host with less than 4 players, everything is working perfectly (without any problems). Of course every time when people joins the same host and there are more than 4 of us, I'm getting kick out. The only way to play fully with 8 players is to creat my own game - but this sometimes take a while (to get 8p). I'm playing B:P online for like 9 months now and I never had something like this! The stange thing is that this happen after I change my antivirus no my pc - but thats probaby coincident... I do not have any problems with other games.

My console is connected with Linksys wrt54g by wi-fi (and pc by internet cable) I opened some specific port for that game, even put my console in DMZ mode and it didn't help. Please help.