Are you on 10.8.2 as ypur profile says? If so, it's worth updating to 10.8.3 which was released last week.
Here's a link to some suggestions that helped some users with similar problems in Lion. May still be relevant to MLion. Also check the links within the article:
Also when I had wifi problems in the past, creating a new network location resolved it:
http://support.apple.com/kb/HT5289 (under the heading 'Making New Locations'.
I've also seen threads where users with wifi problems have successfully solved them by changing the channel in the router's wifi settings.