and the cause was low signal levels from my ISP.
It only showed up in the evenings, when everyone and his dog go online. The modem would look as though it was connected, all the correct lights, but it actually wasn't.
The proof of this was in the powering down of the modem, not the D-Link, and its inability to reconnect once it was powered on again.
The modem worked fine at all other times!
I use an Apple Extreme as my router. It was purchased in December, 2009 and worked fine until March of this year. It is connected to my Mac Pro via ethernet and there is a relatively new iMac that accesses the internet via the airport through the same Apple Extreme.
In March, we lost internet connection. I spent an hour on the phone with a representative of our internet provider. After much experimentation, we narrowed the problem to the router as the modem and the internet connection were fine.
After getting the AE working, it went AWOL once a night for no apparent reason. I have spent hours on the phone with Apple technicians who have patiently had me adjust, check and run a diagnostic test on the AE. The last contact had me reset the Apple Extreme to the original factory settings and it has worked better, although every few days (always in the evening), it will lose internet connection. I can get it back but it is frustrating and there is no logical reason for this to be occurring. My old Lynksys never did this and it was a 1/4 of the price of the Apple product. Has anyone else encountered anything similar?