Computer reboots when modem is on

[Cosaides]

Lately I've been having a weird problem with my computer rebooting on its own. It only does it when my cable modem is on and at random intervals. If the modem is off, the computer stays up for as long as the computer's on. I'm using Windows XP Professional. After a while, the rebooting becomes more frequent, and I have chkdsk run on the next reboot. After chkdsk runs, the problem seems to go away for a while, but eventually returns. Now the fact it only happens when my modem is on tells me there's nothing wrong with the hard drive itself, but the fact that it goes away for a while after running chkdsk, tells me there is something wrong with the hard drive. Anybody know what's going on? I use a firewall, an antivirus, a spyware detector, software to remove all my obsolete files, my hosts file is locked down to prevent hijacking, and I make sure my registry is clean. I can't think of anything that would cause this. Thanks for any help.