What HW version is your router?
What FW version is currently loaded?
Things to try:
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off QoS options.
Turn off Advanced DNS Services if you have this option under Setup/Internet/Manual.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
What wireless modes are you using? A and B modes aren't used much any more.
Try single mode G or mixed G and N?
What security mode are you using? WEP, WPA or WPA2? Preferred is WPA-Personal. WPA2/Auto TPIK and AES.
What wireless devices do you have connected?
Any cordless house phones?
Any other WiFi routers in the area?
Turn off all anti virus and firewall programs on PC while testing.
Turn off all devices accept for one wired PC while testing.
I've had this same problem with my router. I actually starting having this problem when I dropped my cable with Time Warner but kept just the internet service. I thought maybe it was the reason for it but I don't lose connection from Time Warner, it's just with the wireless portion of the router.
I was lead to believe it was possibly my router was getting old and needed to be replaced, so, I went out and bought a brand new one (dlink 655) with the hope these things might change. It worked well for a while but then it started doing the same routine as the old router.
I'm close to leaving the dlink family and going to another brand as well but I really don't want to. I've gone with dlink now for about 8 - 10 years now and have been happy with them and have no real issues up until now. 
I will try the ideas in this thread but seeing dack70's replies doesn't leave much hope with me already. I wanted to reply to this thread in hopes there might be a solution that has come about since June.