Hi, i've got that board at the moment and i noticed that it has/had some strange power/reboot issues. i also noticed it with the same model board on another system. Most of the reboot issues stemmed from exiting the bios. it would start to reboot, and would just sit there at a black screen. all the lights and everything were on, and it sounded normal, just no boot.
I have also seen it act up on a restart from within windows, doing the same thing.
The board seems somewhat picky on its memory timings. Once you get it dialed in and running, it is a fast reliable board. Overclocked or not, once you get it tuned in and stay out of the bios it boots more smoothly.
I think the boot problem stems from a power issue with the board. i noticed that sometimes when i had this problem that the light on my usb webcam would come on and stay on, and the system would just sit there. Normally it comes on, goes off and boots.
As for your problem, make sure you have a good power supply and i would look at going to a nic line hookup if that modem supports it, or a different modem.
Also as tmx468 suggests, try booting and rebooting with the modem unplugged to see if that helps.