wlan stalls with bittorrent

[muno]

Simple lan/wlan setup.

dsl modem->ap->clients

I can transfer huge amount of data on lan no problem, the connection doesn't choke. I can transfer huge amount of data over http no problem.
But if I launch bittorrent, the connection starts to choke every minute or so. No matter how fast data gets transferred over the internet, it just stalls the connection totally for about 20seconds. The computer sends packets but doesn't receive any, during this time the connection drops from 54mbs (perfect signal all the time) to 1mbs and finally sometimes drops to connection alltogether.

This client is set on dmz on the modem, upnp is enabled and there are 10ports assigned for bittorrent.

The connection isn't choked by isp (not allowed), I can use bittorrent just fine on my desktop which is connected via cable.

I think earlier there weren't any problems when I was using some 1000year old orinoco 802.11b card (the ap was different, same brand, but different model).

WPA is enabled, SSID transmission is disabled and mac authentication is disabled.