Drops internet connection if left for an hour

Hi, we're using a refurbed C7000-200NAB modem/router combo. (This was a replacement for an earlier C7000 that was having the same problem. I can't even find a phone number for Netgear support anymore, and they refused to give me a warranty or any time frame for free support on this replacement... so I'm hoping someone here can help please :-)

The problem is, if the computer is left on without being used for more than a half hour or so, the internet connection drops, and I have to restart the computer to get it back. This is not a major problem, but is time consuming and kindof annoying (especially since this refurb was sent to correct the same problem).

For starters, we are not using any wifi- just an ethernet connection. Have checked the cables, contacted the cable company, done several system resets, etc. But the problem recurs. Closing and reopening the browser & mail programs doesn't help, only restarting the computer.  All computers on this system are macs, btw. 

I am the 'more' computer savvy person in our household, but still not well versed in router-ese. I wasn't able to follow the directions on other posts re dropped internet connection. So if you have any ideas for us, it will help a lot if you can oversimplify and spell things out please :-)

Thanks very much,

MJ