First, let's make sure that the service is not having issues. When you look at the following page, are both boxes at the top of the page green? http://192.168.0.1/#!/home/status
Do you see all green check marks on the right hand side in the System Summary and System Status boxes?
Edit: In addition, if you are connected to the modem via WiFi, do you have a device that you can connect directly to the modem with a LAN cable? If so, please try that and see if there is any difference with that LAN connected device.
2nd Edit: Forgot to add link to page I was asking about. Thanks, jezra.