Wireless speeds slow

I have a Netgear WGT624 super G wireless router. My problem is that my wireless networking is very slow. It's great at first but after a little time it slows to a crawl. The signal strength is still very good to excelent range but a 108Mbit connection will go to 1Mbit. I have found that going into router setup and releasing the IP and then renewing it will help. I don't understand it doesn't actually change the ip but it works. There is something called a lease. It is good for 3 days. It lasts in practice about 3 hours. Then everything slows to a crawl. If I release and renew then usually it helps but not always. I'm new to the wireless game and it's driving me nuts. I know radio is unreliable as a source but the disparity between advertised speed and reality is pathetic. Super G is 108Mbit I hoped for 54Mbit. Typically I get 18Mbit on a good day and everything is working right. I have another router that I'd like to use to extend the network but I don't know how to do that and the link for the FAQ to explain that doesn't seem to be working. Right now I just want to get the wireless running as fast and especially as reliable as possible.

Well I tried tinkering with the channel a bit but I didn't think it made much difference. I'll try again. I live rurally and we all have 3 acre lots. So the closest house is at least 700ft away. I have a 2.4Ghz phone in the same room as the wireless router. I really don't know how much interference it could cause. It has a censor on it that allows drifts in frequency as well to avoid conflicts with other devices. ie other phones wireless speakers etc....I will switch it out with a 900 MHz phone I have instead. Can anybody tell me how to use the second wireless router to extend my network? Would it be easier to just go buy an extender? Do they really work? I think if I use an extender of some sort it will work out better because I'll have better signal.