I notice that ping -l 732 137.118.216.1 works well
while ping -l 800 137.118.216.1 fails
suggest you change the MTU on your system to 732
here's how
If you set the MTU size for a network interface manually, this setting overrides the default MTU for the network interface. The MTU size is the maximum packet size in bytes that the transport will transmit over the underlying network.
This method affects packets sent to all destinations and may significantly affect the performance, depending on the MTU size that you set.
To set the MTU size for the network interface, follow these steps:
1. Click Start, click Run, type regedit, and then click OK.
2. Locate the following key in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\<ID for network interface>
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type MTU, and then press ENTER.
5. On the Edit menu, click Modify.
6. In the Value data box, type the value of the MTU size, and then click OK.
7. Quit Registry Editor, and then restart the computer.
Be sure to tell them all the things you tried and about Jobeard's tips. Just because they gave you a new modem, it doesn't mean that it is good. Have them swap it out again