packet loss with ndis wrapper

hello everybody

i have some problems with the wlan interface of my new ibm thinkpad r50.
the notebook is running freebsd-current(uname: FreeBSD 5.2-CURRENT Thu Apr 1).

the wlan card is a builtin intel pro/wireless lan 3b pci. i compiled the if_ndis module following the instructions in ndis(4) and ndiscvt(8) and using the winxp drivers from the ibm homepage.

the card is recogzined successfully(dmesg | grep ndis0):
ndis0: <Intel(R) PRO/Wireless LAN 2100 3B Mini PCI Adapter> mem 0xc0214000-0xc0214fff irq 11 at device 2.0 on pci2
ndis0: [GIANT-LOCKED]
ndis0: NDIS API version: 5.1
ndis0: Ethernet address: 00:0c:f1:12:ab:b1
ndis0: 11b rates: 11Mbps 2Mbps 1Mbps
ndis0: link up

my network consists of 
192.168.0.1 (server)
openbsd box providing internet access via NAT.
(caching nameserver, dhcp, pf)

192.168.0.2 (wlan router)
netgear MR814 wireless router.
(i deactivated the builtin firewall, dhcp etc because openbsd shall handle this)

192.168.0.3 (local1)
just a local machine ..(freebsd 5.2)

192.168.0.32 (notebook)
thinkpad r50

basicly everything seems to work fine, as soon as i set ssid i can start dhclient ndis0. http works, but all connections that need to stay alive longer are very unstable (ssh, irc, icq). the connections drop because of ping timeout or have very long (>60s) lags.

i also noticed the following when doing some ping's:
ping local1 -> wlan router 0% loss
ping local1 -> notebook >85% loss
ping local1 -> server 0% loss

ping notebook -> wlan router 0% loss
ping notebook -> local1 0% loss
ping notebook -> server 0% loss

ping server -> wlan router 0% loss
ping server -> notebook >85% loss
ping server -> local1 0% loss

now another strange thing: when i start a ftp client to get a big file from the server (speed sometimes 170kb/s, somethimes 580kb/s), while downloading the packet loss disappears completely !
ping server -> notebook 0% loss
everything works fine then, so as long as i'm permanently transfering things i can stay on irc / icq without lags, but as soon i stop the transfer the connections starts to get laggy and unstable again.

it shouldnt be a problem of range because i'm in the same room with the wlan router and i plugged out all phones to avoid interference.
i also tried to plug out the openbsd box to use the wlan router for all the dhcp and internet gateway stuff, which worked fine but didnt solve the problems mentioned above.
i tried different wlan channels but the only thing to keep connections stable is to transfer data from anywhere to the notebook.

if anyone could help me to find out whats wrong there i would be very happy. if i missed some, please ask and i'll try to supply them.

thanks in advance
sebastian