I am having a very similar issue. After upgrading my firmware to 1.13 I can no longer connect to Xbox Live with a solid connection speed. If I plug directly into the DSL modem, I can play on Xbox Live with 4 green bars of connection, no problem. The best I can get when playing through the router is 2 yellow bars. I have tried everything I can think of to see if it will help and nothing has worked. (on a side note, moving the Xbox 360 into the DMZ actually made the problem worse. It would connect to Xbox Live, but not to a game host.
I have since gone to version 1.12, but it is having the same issue. I did not have this issue with the older firmware version I had (I think it was 1.02 but I can't be 100% positive). Lucky me, firmware 1.12 wont let you go back to an older version.
I noticed in the previous post it stated that version 1.12 and 1.13 of the firmware are having issues with UPnP. How soon will that be fixed?
I would think a firmware issue with UPnP would be a high priority to get resolved.