As someone else said, it'd be useful to know if it's the wireless side or the internet side that's dropping out. I assume you've got an ADSL filter thing on all your phone sockets?
In the original post, there is mention of the ISP side looking OK.
IP address conflict could be telling - perhaps the WiFi isn't very secure, and somebody else is, um, using it.
Quite difficult to diagnose WiFi issues, though, truth be told - using a wired connection may help diagnose, but only if it's usable for some time when you could expect problems to occur.
Personally, I haven't got loads of experience with lots of different routers, I do tend to use DD-WRT on the ones I use, though, and find the site survey and the WiFi network status sections very helpful.
If I had any advice it would be have the line checked, then try and rule out hardware. After that, setup a new WiFi environment with hidden SSID, decent security (WP2 AES) with decent key length, and MAC filter list only allowing the devices you've specifically configured - then hopefully you can remove outside "usage" as being a factor. That and site survey type functionality to try and establish which parts of the WiFi channels you can use without interfering / overlapping with any neighbouring WiFi networks.
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