could be a number of issues as we have already established what model is the netgear router and whats the wifi cards used to connect the other devices or are they all internals if so try to find out what they are i.e Broadcom, Airport ect.
to get a little bit more detailed info on the other broadcasting networks around you use netstumbler it will tell you what channel they are all on and will allow you to see if a channel range isnt being used that you could try to cut the interference. www.netstumbler.com
Also not a bit fan of WEP you router will have a thing called ACL list on it this is a list allowing you to add the MAC address of each wifi device you allow to access the network this is like putting a man on the door and saying if you name aint on the guest list you aint getting in, WEP can be hacked on its own keep it active as it also encrypt your data something that ACL on its own doesnt do.
MAC address filtering is no longer considered a security precaution, anyone who wanted in would find it no problem at all to overcome (spoofing a legit address) and it just adds hassle to administer e.g. a friend wants to use the connection etc..
The best you can do is go for the highest encryption level that your least advanced device supports e.g. if all devices support WPA then use WPA, if you've a device that only does WEP then lower to WEP to accomodate that device.
Don't bother to hide the SSID, again it's no security precaution and it's contrary to the standards for 802.11a/b/g networks, again it just causes more issues than it resolves.
Bottom line, if someone wants in they'll get in, all you can do is make it inconvenient so they'll find another easier target more attractive.