Am having a similar problem. Am operating N-mode only. Wifi connection seems to drop during download operations (even small files on the order of 40KB) on Win7x64 HP and Win VISTA laptops. Wire-connected PC not affected. Other wireless devices (iPhones, iPod, IP camera, 2xRokus) also not affected. The two laptops do not exhibit this problem on other networks. The dropped connection can always be restored, but it's a pain. Have tried everything mentioned in the other posts here. DIR-655 Hardware Ver A4, Firmware version 1.32NA. If I do a firmware upgrade check from the UI, it says my version is current, even though the website says 1.37 is current for A-version routers. This has actually been occurring for some time (2 years maybe), I just haven't gotten around to doing anything about it.
DIR-655 is set for Channel 11 on the advice of Roku Tech Assist (ROKUs would disconnect and be unable to reconnect; setting Router for Ch 11 as recommended resolved that). This problem actually preceded the Roku thing which was less than one year ago.
Different but perhaps related problem; router wireless radio will occasionally (once or twice a month maybe) just drop out. Going into the UI and turning wireless off, then saving (router resets), then turning it back on & saving again, usually restores wireless operation. Once or twice I've had to unplug the router and plug it back in again to get the wireless back.