Airport Extreme and BitTorrent

I seem to be having major issues with BitTorrent speeds, and all the troubleshooting I've done so far has led me nowhere.

I have a brand new Airport Extreme, with a MacBook Pro connecting wirelessly via 802.11n on the 5GHz band. Signal quality is excellent.

I'm using uTorrent, and have it using UPnP. When downloading torrents, I can see that the machine accepts connections (green light in uTorrent), but the speeds hover around 20KB/s. If I go to uTorrent preferences and click on "Randomize Port Now", the speeds jump to 400-500KB/s, but then trickle back down to 15-20KB/s after 5 minutes. I have also tried disabling UPnP and configuring port forwarding directly, and the end result was the same -- high speeds initially, and slowly dropping to a crawl.

Is there an issue on Airport Express? I have not had this problem with my Netgear 802.11g router two weeks ago. The ISP is AT&T DSL, and they are not throttling BitTorrent traffic. I looked all over the advanced settings in AE, but did not see anything that may contribute to this behaviour.

Any help would be appreciated -- I still have a day to return the AE if it's an issue on the base station.

Thanks in advance!