What Hardware version is your router? Look at sticker under router.
What Firmware version is currently loaded? Found on routers web page under status.
What ISP Service do you have? Cable or DSL?
What ISP Modem do you have? Stand Alone or built in router?
What ISP Modem make and model do you have?
If this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems.
Some things to try:
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual? You can find these under Status/Device Info/Wan section.
Turn off ALL QoS (DIR only) GameFuel (DGL only and if ON.) options. Advanced/QoS or Gamefuel.
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. Setup/Networking
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?
Try single mode G or mixed G and N?
What security mode are you using? Preferred security 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? Use InSSIDer
to find out.
Turn off Short GI and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Also need to review the DGL-4500 forum for the XBL sticky.
I would recommend that you get a DGL-4500 router instead of the 615 as the 4500 is better and handles gaming with XBL and PSN very well. One problem with the 615 is that it doesn't have extended QoS options to set up as other D-Link routers do. So your kind of limited on the 615. The 4500 and other D-Link routers have better QoS options to set up and use.