Most likely, the cause of spontaneous reboots is the increased temperature of the modem. Take the modem in hand and check whether strongly it is heated. Try during work to send to the modem the fan, or put it in a cool place, for example on the window sill.A second possible factor is the lack of voltage. Check the power supply of the modem for damage, check its technical characteristics. Perhaps his power is not enough to supply modem power.A third reason for spontaneous reboot may be outdated modem firmware. Check for current software for your modem on the official website of the manufacturer. If you have an older version installed, please update it.Problems on lines of communication can also affect the modem is not the best way. Line faults can cause conflicts on your modem with the equipment of the provider, with the result that it is possible to observe a spontaneous reboot of the device.The fault of the hardware of the modem may also be the cause of these reboots. Try connecting it to another computer and check. Malicious software (viruses) can also cause a spontaneous reboot of the modem. Check your computer for viruses.It should be noted that an unsuccessful firmware update may cause malfunction of the modem. Before you perform this operation, carefully read the documentation of the modem, and better contact the service center.If the above tips do not help, you should contact the experts.