I've had my DIR-655 for a couple of years and through that time I've had periods where my wi-fi connections will randomly drop, and numbers of months where the connection has been stable. I'm in one of those periods where it drops, and it does so often, every 5 minutes or so. When it drops off, it's no longer broadcasting because I can no longer see it in my list of available networks.
I'll try to include all pertinent information, but let me know if I've missed anything.
What ISP service do you have? Cable, but I've experienced drops with both.
What ISP modem do you have? Arris Touchstone Telephony Modem TM602G/115. When I ran off DSL, I had the same issues with my DSL modem.
What wireless modes are you running? Mixed 802.11 n and g
Any cordless phones in the house? Yes, a decent distance away though
Any other WiFi routers in the area? No
Setup Reserved (Static) IP addresses on the router for your devices. Done
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual. Done
Ensure Advanced DNS Service is disabled if you have this option. Done
Ensure DNS Relay is enabled. Done
Also I have UPNP disabled. Haven't disabled QoS yet. I'm hoping that's not the issue, I really don't want to disable it.
Load doesn't seem to matter. Cabled connection work fine during periods of disconnect, though once in a while they will drop.
Not revision B router and running firmware 1.35NA. I've experinced the same drop issues with previous firmware versions.
I'm running a varitey of devices: Windows 7 laptop, Iphone, Android, Windows Vista desktop. While typing this on my laptop, I've lost connection 4 or 5 times. fun fun!

I think that's it, but let me know if you have any further questions!