THIS IS TOO STRANGE. I had this problem as well. ATT/Motorola Surfboard. My problem has ceased. Maybe it was an ATT thing. Maybe it was these solutions I tried.
1) Plug the modem directly into the wall if it is in a surge protector now.
2) If you are using Windows XP, left click on start/control panel/network settings/Local Area Connection/properties/configure/power management. Make sure the "allow the computer to shut down this device to save power" box is NOT checked. Click all the OK's and close's, then restart the computer.
Since I have done this today I have had ZERO problems
1) Plug the modem directly into the wall if it is in a surge protector now.
2) If you are using Windows XP, left click on start/control panel/network settings/Local Area Connection/properties/configure/power management. Make sure the "allow the computer to shut down this device to save power" box is NOT checked. Click all the OK's and close's, then restart the computer.
Since I have done this today I have had ZERO problems