Linksys WRT54GS Routers Drop Wireless Constantly?

I have been having a lot of trouble lately with my wireless network. Basically, under certain situations it will constantly drop out, by which I mean all computers will lose connection with the wireless network for about 30 seconds and then reconnect.This is happening with two different wireless routers so I don't exactly know what is happening. I usually use a Linksys WRT54GS v7 but after this started happening I borrowed a friend's Linksys WRT54G2 v1 router to test it out with no change. I also updated both router's firmware last week, and changed both router's channels to 1 in case that was a part of the problem. To clarify,only one router is being used at a time, I have just tried different steps with each one.The other day when it was happening repeatedly, I connected one of the computers directly to the modem and had no problems so I do not think it has anything to do with my modem. On the network, depending on the time of day, there will be any combination of three macbooks running the latest version of snow leopard and one older PC running Windows XP. The network can drop out with only the PC, no PC, or any combo of macs. There can also be two iPhones and a Blackberry using the wireless depending on who is home. The most consistent problems seem to occur whenever something is being uploaded. We seem to get major issues when my wife is sending relatively large files (5MB+) using her macbook's mail program (either sending through gmail or her work address). However, yesterday it was happening every 30 seconds when someone was talking on skype on her macbook while there were two other macbooks connected to the network but doing nothing more than using a browser for basics like google.I have also noticed that this will often happen, though usually only once rather than repeatedly, when a computer that hasn't been on is turned on.