You're not going to get gigE speeds over wifi without some additional investment. When you hook up your PC to the modem with Ethernet you should be getting the full speeds or close to it at least. If you're interested in wifi at gig speeds though I have a solution for that is working.
Intel 92xx wifi card ~$25
Netgear X4S Router ~$200
I have my own modem and don't subscribe to renting one from CC since the rental fee pays for the modem costs in little over a year and they last forever. I went with the MB8600 and it's been rock solid. If you have anything besides tv/internet you'll likely need some additional box from CC to keep them along with the $12/mo for the rental fee.
Ok, that changes things a bit but, you did mention a wifi router.
I never rely on the CC equipment for HSI ever! It's usually the cheapest thing they can purchase in bulk.
Double check your Ethernet adapter is in Full Duplex mode and not 1/2... most gigE adapters though are either auto / full duplex at 1Gig. Depending on how long ago the drivers were issued they might need to be updated as well depending on the adapter.
Then I would first start out with a hard reset using a peperclip. I would grab the speeds again after that. Also, grab the signal levels by logging into the device at http://10.0.0.1 or http://192.168.100.1 when directly connected to it. Depending on what the readings are you might try moving the XB to where the Coax comes into your house and getting the readings there as well to see if there's indeed a wiring issue or a bad splitter somewhere in the path.