Damien,
A question....
Can kind of router are you using?
Iif the router is making these changes on it's own I would check the manufacturer's web site to see if there are any postings regarding this problem and firmware patches/upgrades that might fix it.
Also check the settings under the Authentication Tab on the Local Connection Properties Box for the WAN card on the Laptops/PC's. There is a option to
"Enable network access control using ieee 802.1X". Make sure this option is turned off before using the 64 or 128 bit, open/shared encryption that the router uses. That causes hipccups and conection problems sometimes between the Wireless card and the router.
Let me know how it goes and good luck,
Carlitosway