Have you changed the wifi channel on the router? It's worked for some.
Here's a catch-all of some things that have helped some people. It sounds like you've tried a few of them, but the links may offer other ideas - in no particular order:
- Check wifi security - if it's WEP change it to WPA or WPA2 (preferably, as it's more secure)
- Change the wifi channel on the router
- Create a new Network Location: http://support.apple.com/kb/HT5289
- Reboot the router
Also take a look at this link and the links within the article for other ideas. They mainly reference Lion, but still seem to be relevant to Mountain Lion:
...and this link: