You need to check your routers WiFi settings as it usually is a problem with WEP encryption.
Best way to attack this issue is to disable ALL security, get a good solid connection and then re-enable security.
The way I attack WiFi security is threfold.
- MAC address access list
- WAP encryption (128 bit)
- Hide SSID
I find that this works a treat.
Basically if the AP can't be seen then it presents less of a target.
The MAC and WAP adds more security without slowing access speeds.
I must admit I have only tried WPA once and I ended up locking myself out of the AP, so I stuck with the above method.
Hope this helps.