A daily reboot, if necessary, will help prevent downtime. However, it will not "fix" the problem.
If you are discussing a combination unit (modem AND wireless router AND/OR switch) that is owned by an ISP, then your plan of action should be as follows:
1. Call the ISP and have them check the physical exterior line. DO NOT allow them inside your home. Make sure they are not going to charge you. Tell them you saw an exposed line. Tell them you will be out of town and they can check the property. If you allow them inside, they will 100% blame you and charge you. If they are outside, it is their problem, but get it from them in advance.
Now that you know the line is 100% good, up to the connection into the residence...
2. Call the ISP and tell them that the modem / wireless router / switch is spontaneously turning on and off. The day before, switch the unit on and off about 4 times. They will see this. At this point, insist that you want a swap and will accept nothing less. They should tell you to come by with your unit, and swap it with another for no charge.
3. Take it back to the residence, connect it, check all cables, connect it directly to a computer (laptop/whatever) with an ethernet cable. Run speedtest.net and make sure you are getting your paid-for speeds. If everything is good, reconnect all wireless devices and all else.
4. If you are STILL having problems, then you have narrowed it down. It is either: Power (electrical) or external to internal wiring (ethernet, coax - inside residence).
-------------------------------------------
As an ideal rule for uptime, you want the modem as a separate device from the router. The router should be simple, with only ethernet switches on the back. Connected to the router should be a wireless access point. Connect as much as possible directly via ethernet.
If you really want to go for it, get an old computer with 2 NIC's, and install pFsense or Smoothwall. The software is free and will outperform any consumer router on the market.
Good luck!