pppd freezing system?

I have been using a usb gprs modem to connect to the internet.
Periodically (often) the entire system will freeze, requiring a reboot.
I can't kill the X server (ctrl+alt+backsp). I can't ctrl+alt+del. I
can't even reach my box using ssh over the network. I have previously
connected using a cable modem attached to the LAN with no such
problems. So far I have encountered the problem only within XWindow,
but it may be that working in command line, I am not putting as much of
a load on the system. Any ideas?

uname -rm
2.6.18-1.2849.fc6 i686

pppd version 2.4.4

Phone: Motorola V360