While changing various settings on the router are valid troubleshooting steps, I recommend everyone having the same problem with the wireless signal dropping, observe the LED on the front of their routers (see my previous post). When I called D-Link tech support, they didn't seem at all suprised and were perfectly willing to replace both of my routers without question.
Despite those in this forum that would tend to disagree... the WiFi LED shutting off completely and the complete failure of the hardware to transmit a WiFi signal when observed through an OS independant stand-alone WiFi sniffer is absolutely a hardware issue and has nothing to do with the configuration of your router or your clients. Even if your router is colocated with another, more powerful, device that is walking on your wireless signal, the LED should remain on and despite your inability to read the signal because of the interference, the router should continue to transmit.
To reiterate, I have 2 DIR-655s in my home network, both are HW v.A4 and FW v.1.35. The first acts as the fully functional router and is connected to a 24 port Cisco Catalyst switch providing Ethernet throughout my house. This router provides the upstairs wireless coverage. The second router is located in the downstairs portion of the house and is acting as an access point only providing a second wireless network for coverage in the downstairs rooms of my house. I also have an AT&T MicroCell which was added ~after~ the problems had already been identified.
The clients my network supports on the wireless side include iPhones, Nintendo DSi's, X-Box 360, PS3, and PC's running WinXP, WinVista, Win7, Debian Linux, and MAC OSX. When I can have multiple device types and multiple operating systems all colocated and watch them all drop signal at the same time and in sync with the LED shutting off, it's not a configuration problem, it's a hardware issue.
So, after you're done chasing your tail with settings that you shouldn't have to change (if the router is made to broadcast b,g,b and wont, that's a problem) then just call tech support and get yourself a replacement.