What ISP Modem do you have? Stand Alone or built in router?
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 under Advanced/Firewall if you have this option.
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.
Any cordless house phones?
Any other WiFi routers in the area? Use
InSSIDer to find out.
Turn off Short GI, WLAN Partiton and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Turn off all anti virus and firewall programs on PC while testing. 3rd party firewalls are not generally needed when using routers as they are effective on blocking malicious inbound traffic.
Turn off all devices accept for one wired PC while testing.
Check cable between Modem and Router, swap out to be sure. I recommend CAT6 and the shorter the better if possible. I use a 1 footer. Swap cable between modem and router. Sometimes these in box cables are not make well.
Ensure the phone lines if on DSL or the cable line is in good condition going to the modem. If on cable, make sure the cable line going to the modem doesn't have any T.V. line splitters on it, or if needed, only have ONE good quality one if needed. Splitters will lower the signal going to the modem. If on DSL, make sure the phone lines are correctly connected and properly filtered.