I was having serious connectivity problems with my G300N (1.64 firmware) and a Nokia N900. Large file transfers would almost always freeze at some point. This problem disappeared when I turned 40Mhz mode off.
But now I have another problem: if the connection is idle for a while (~5min), the router drops the connection. I get the following message in the router log:
ra0: had been aged-out and disassociated : ec:9b:5b:xx:yy:zz
(ec:9b:5b:xx:yy:zz being my device MAC address)
For me, it is not a NAT problem, as I'm using the the router in bridge mode.
As a temporary resolution, I've shortened the DHCP lease time in my LAN so that the N900 has to renew it's IP lease every 4 minutes. That seems to work so far.