Bob,
It is very unlikely the router is locking up. When it happens you should logon to the router from your LAN and see what the WAN status is. Also, set yourself up with InControl2 as it has a log of the up and downtime of the router.
Most likely the problem is the handshake between the router and the cable modem. This has been a long onging issue with Peplink routers and some cable modems. For example see this
You can also go into the cable modem and look at its log to see when it goes off line and online. This will likely confirm the problem is not the router itself failing but the communication between the router and modem failing after the modem has gone offline.
One solution is to set the WAN definition to always use full duplex and always run a gigabit speeds and thus avoid the problematical handshaking. It worked for me once, cant promise it will work for you. The article has other ideas too.