Can't Join Xbox Live Party?

Alright so lately I haven't been able to join my friend's xbox LIVE party or games, however we can join each others games when there are more players(an example would be he can't join my BFBC2 squad in the MP menu but he can join after a match has begun). Also I've noticed that this only happens when I play connected to my Belkin G Wireless router (F5D7234-4 v3 to be exact) but when I play connected directly to my modem everything is fine, no errors no disconnections it works the same as it did before(NOTE: I had the router and used it before all this started to happen). We've both properly port-forwarded our routers to the xbox LIVE ports required and now he can play with his other friend. Both our NATs are open so I'm sure that isn't the problem. His router is a new model by Thomson (TG782T to be exact again). Is there probably any setting causing disturbance? Is there probably an uncompatibility with our routers? Also I'd like to add that my friend doesn't have a modem, his internet company(which is different to mine) set up the connection directly to the router. Also both our routers have the latest firmware, our Xbox IPs are in the router DMZ and UPnP is enabled. Any and all help is appreciated, this problem is starting to get annoying since I'm forced to use the router since my sister has a netbook that needs to use wi-fi, which she uses non-stop. And another little question I have, in my routers config page on virtual servers, does the 'private IP address' need to have my computer's IP? or the IP of my xbox? I ask that because my friend has been able to connect with his other friends after port forwarding but I'm not able to and I'm not sure if the private IP he set was either his xbox IP or computer's, and he can't remember either.

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i didn't feel like reading all of that but trust me…i'm a doctor

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