If the modem isn't yours, then you should not have to pay to replace it if it is indeed bad. If it turns out to be something in your network and not Comcast's modem or external wiring, they could charge you for the truck roll.
If 10.0.0.1 is the internal (LAN) IP address of your modem, from one of your PCs (assuming it's windows), open a command prompt and run:
ping -n 300 10.0.0.1
Then post the summary output here in this thread, this part of the output:
Ping statistics for 10.0.0.1:
Packets: Sent = 3, Received = 3, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 2ms, Average = 1ms
You should not see packet loss over ethernet to a device with nothing in between.