You really need a good troubleshooting approach now to find out what the cause is:
1) Take the router prone to rebooting, have it turned on but not connected to anything including the telephone line, does it reboot now? If Yes, it's likely some interference coming in from the mains supply or a rather odd mains supply problem, or could be RFI from the air.
2) If no reboots, you can consider the power side fine, so now connect it to the telephone line and let it sync up, but connect nothing else and do not use the connection, does it reboot now? If Yes, then likely EMI coming in from the telephone line or some voltage that shouldn't be there.
3) If no reboots, connect it up to your network switch and start connecting things back to it, one device at a time, until it starts rebooting, you then have identified the problematic device.
No one can give you any other helpful advice really without the above steps taken to find the cause, otherwise it's all guess work.