I had similar problems after upgrading to 10.8.4. I used the Wifi Diagnostics tool in 10.8.4 (press the Alt key together with clicking on the Wifi icon at the top right of your screen); this reported that the were a number of nearby wifi stations using the same channel as my own router. I then installed the Airlock Dashboard Widget from the link below.
This confirmed that there were a number of strong wifi stations nearby and showed their channel numbers (many of which were the same as used by my router). It was then simply a matter of setting my router to use a channel that did not conflict with any nearby routers. This has resolved the problems I was having...... So far! I speculate that 10.8.4 may have improved the detection ability of my iMac and, as a consequence, co-channel interference became a problem. It's very quick, easy and safe to do what I did and, if you try it and it doesn't help, you haven't lost anything.
http://www.apple.com/downloads/dashboard/networking_security/airlock_danielpimle y.html