Is the cable modem a stand alone cable modem with out any built in router? Been seeing some new cable modems come with routers now. Usually they don't.
Are you sure you are seeing your ISP speeds while connected to the router? Do some speed checks at speedtest.net to be sure. If your on cable, make sure splitters are kept to a minimum on the cable line. I recommend not using any if possible.
Ensure all the cables are good and connected to the correct ports on the back.
Does this happen if you use factory settings? Do a factory reset from Tools/System.
Does this happen with just 1 xbox online?
How are the gamefuel setting set up? Make sure they are correct and with out any typos. Are the xbox set up for static IP addresses on the router?
If this all correct, make sure the xbox network settings are set to automatic.
Ensure that primary and 2ndary DNS ip addresses are being filled in under Basic/Internet/Manual. You can find these in Status/Device Info/Wan section if they are not.
MTU should be at 1500.
Ensure that Dynamic Fragmentation is OFF.
Try setting the manual upload link speed to 2048 or 4096.
Ensure that Firewall TCP and UDP is set for Endpoint Independent for both.
Ensure uPNP is enabled too.
Do a Configuration file save to back this up.