Why does my Macbook randomly drop my Wi-Fi connection, while the network is still in the list of available networks?

As stated, my Macbook Pro tends to drop my Wi-Fi connection to my home router (which is a few feet away). I check my networks, and my home network is listed as available. I click to connect and everything works fine afterwards. But only hours later, the same thing occurs!

The funny thing is, I could return to my MBP and fire it up from standby, and I can browse the web without issue. But, out of nowhere, I begin receiving the familiar “page not found” messages.

So, the connection is not dropped when I go to standby, but occurs minutes after resuming my activity usually.

I have Mountain Lion on a 2011 MBP, and the router is a D-Link 655 which was purchased in July.

Note, this same thing happened with a Belkin router I owned previously.

Thanks for any ideas.