Hi hows things
Although Im not shure what causes it, it is a configuarble feature.
The polytech that I go to were testing a cisco AP late last year and had the SSID set to a rolling on/off state. This means you suddely detect a network and then lose it again without actualy connecting to it. They had this in place so that they could test the range of products, through walls and around campus without people actually connecting to their network.
I have had that also happen on my home wifi network but am unsure as to how I created it so a good old reinstall driver solved my problem.
If you run netstumbler it may alow you to diagnose the problem further as it should constintly detect the wireless signal.
Try setting properties on your D-Link wireless device to, auto channel, atuo negoiate, and auto speed control. (Terms may very on your product as I use USR AP's and cards)
well after all that im not sure if there is any helpfull information in there.
good luck