I started having the wifi problems under Snow Leopard, was about ready to upgrade to Mountain Lion anyway, and thought the upgrade might solve the problems - strong networks in the house appearing and disappearing, connection timeouts when trying to join, etc.
The upgrade didn't help; I had the same problems.
My setup is an Asus RT-N56U (excellent, fast router) with Network1 on 2.4gHz, and Network2 on 5gHz. I also have Network3, a Linksys WRT160Nv3 running DD-WRT set up as a repeater bridge, linked to Network1.
Connecting to any network had problems, but it was a little better connecting to Network2, so I thought there might be a problem with the others.
I downloaded a new copy of the firmware for the WRT160N and went through the process of setting up the repeater bridge. When done, all networks worked fine, better than ever, fast, no dropouts - fixed! Just tinkering, I enabled UPnP service on the Linksys WRT160N.
Bad move. All problems returned. I just went back in and disabled UPnP on the Linksys WRT160N and now everything is fine. It's been a couple of hours, and no problems, just fast reliable wifi. Hoping this might help others solve the maddening problems. Good luck!