Hi bewareofthecow,
Do you have a Wireless Telephone Base near to the Router or next to the Router?
Instead of allowing D-Link to autoscan the Wireless Channel Scan in Wireless Settings, uncheck Enable Auto Channel Scan and keep changing the Wireless Channel and also limit the highest Transmission Rate that the Router broadcasts at, to that of the slowest device on your network that you always share files with.
Example: for Transmission Rate:, mine is set for MCS-11-52 (108) Mbit/s.
I have a NetGear Storage Central Unit that its highest speed is 100 Mbit/s per second, wired to the Router.
Transmitting at a higher speed then the highest speed of your slowest device, will cause frequent Wireless loss.
By limiting the internal speed of the Router, to that of your slowest device, improves reliability of the D-Link Router.