I was having a very strange problem and I believe I finally resolved it. I wanted to post this here in case other people run across the same issue... you know, to add the the collective knowledgebase.
My new Mac mini (late 2012) was working well, except for the fact that the WiFi would occassionally basically stop functioning. It would show as connected, but it would be incredibly slow to the point of basically being stopped. This would sometimes last for a few minutes... sometimes for an hour or two or five... then it would be fine for a week or two until it started over again.
No other devices on my wifi network (iPhones, iPads, other computers, my TiVo upstairs, etc) ever had this issue. Only this Mac mini.
One day when it was working I called Apple and worked with them. We trashed network preferences, etc, but it didn't seem to help. They had me boot from my recovery partition and it was still happening. Since this is a completely different OS boot, which ruled out a software issue. So it was either Mac mini hardware, or the router hardware. I ended up manually configuring the router to use a different channel... which seemed to help for a while, but then it came back again.
I bought a little Wifi analyzer program from the App store to see if there were any interferring networks. There weren't really, and when some rarely showed up they were very low power. I ended up discovering that, when there were problems, the signal level didn't drop, but the noise level jumped significantly... I looked around at what could be causing noise... and, AHA!, my external USB backup hard drive sitting right next to the mini. Now, I only noticed this when the signal was already working (seeing that now the noise was a lot lower than it had been when I was having the problem) - so I don't have direct evidence that moving the hard drive away lowered the noise, but I haven't had a reoccurance in the week since I moved the hard drive about a foot or two away from the mini.
Further, I found this:
http://support.apple.com/kb/HT5172 (see "Sometimes I have difficulty..." at the bottom)
So... if anyone out there is having intermittant Wifi problems... take a look at see if you have any USB 3.0 devices, especially hard drives, right next to your machine. Hopefully this might save you the weeks of confusion I went through.
(If my issues reappears despite this fix, I'll update this posting, but so far so good.)