I have the exact same issue!
10.8 (12A269) (C2QGX09BDTJW)
1.6 GHz i5
2GB 1333 MHz DDR3
Macbook Air 11"
My wifi connection was also dropping every few minutes. I could expidate the drops by streaming a couple videos at the same time. Snow Leopards wifi was never perfect for me either (it would drop about once a week) but it was stable. Upgrading to ML made it so much worse.
I had found a solution that made it completely stable (see below) but still looked for a proper fix:
Enable WLAN: Yes
Mode: IEEE 802.11g Only
Transmitting power: 100%
Enable WMM QoS: No
Not having EXACTLY this brought the problem back.
Finally, after already checking if there was a firmware update for my router by going into its configuration and it telling me it was up to date, i wondered if my router was wrong. Sure enough i went to the dlink website (i have a dlink dir-615) i found a new firmware version had been available for the past 2 years.
After updating the routers firmware my wifi is running so so smooth with whatever settings i choose.
Hope this helps!