Do you have both WiFi and Ethernet connected devices?
From the WiFi device, not being able to ping the router rather fits an issue I've seen before with the 2704n routers - WiFi connectivity becomes limited, I suspect at the time the PC needs to renew its DHCP lease. As you report rebooting the router fixes everything.
You might be interested in digging deeper here.
Log the time you restart the router and the when this happens - that will give you an indication of the up time to failure time duration.
In a command prompt, run ipconfig /all and note the lease start and expiry time - at half way between the two (half-life), the lease should be renewed ... keep a watch on this ... if you find that the lease has not been renewed at the half-life, watch out for the WiFi connection to fail when the lease expires.
This is rather speculative, but is what I think is happening from evidence I have seen elsewhere. I am not in a position to use a 2704n to verify this for myself.
HTH?
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