I was going to say, if your seeding out 300, I don't think you classify as a "light" user. Heh.
As for the number of seeds, I have a idea. It could be, as the seeds go up, so does your transfer rate. And as you reach past a certian transfer rate, the router changes how its transmitting the signal. Think about it like this. You have a cell phone, you just talking to someone and have 5 bars on a 2g connection. You go to download a site on the phones browser, the phone swaps to 3G to get you faster speed, but you only have 2 bars at 3G. I found this too from good ol' Tom's hardware.
http://www.tomshardware.com/forum/20446-43-wireless-networking-general
What does the Basic Rate setting in an access point or wireless router do and how does it differ from the Transmit Rate setting?
The Basic Rate set is the rates that all clients that want to associate with a given access point must support. For 802.11b WLANs, these will be 1, 2, 5.5 and 11Mbps. This information is transmitted by an access point as mandatory rates in the Supported Rates element of various management frames.All current-generation 802.11b products support the 1,2,5.5 and 11Mbps basic rate set. However some very old 802.11b clients may only be able to associate with APs advertising a 1, 2Mbps basic rate set. This is why some APs allow you to change this setting.The Transmit Rate setting is used to set the fastest rate that an AP or wireless router will send data. It can be used to force a lower rate in order to trade off speed for more reliable connection in WLANs where many clients are operating at low signal levels.
It would take alot more research to understand the exact mechanic behind the change, but I think thats basically it. So a easy fix would be to see what rate you have when you begin, and the lock the router to that.
Router > Wireless > WIRELESS NETWORK SETTINGS > Transmission Rate