Instability with WRT54G

Hi, I recently bought a Linksys WRT54G to use with
my Prism based PCI card (XG900 clone 1260:3890 17CF:0020)
I use firmware 1.0.4.3 .
When I do only web surfing, all is running 'well' but if
I start big file transfert, the speed is very low and
there is many disconnection to the AP.

Is there somme know problem between Prism and Broadcom
based card ?
(I have an Atheros based card with no problem on the WRT54G)

Does anibody extracted 2.4.6.0 firmware from the big .SYS
in actual Windows drivers ?

Seb.