Computers Randomly Cause Internet Outage

"The internet would be available for about 15 minutes then it would be gone..."
Does this mean no traffic at all?  Or just web traffic?  Could you still ping a server on the Internet by host name?  Could you still ping a server on the Internet by IP address?  

"If we reboot all the computers it will work for a few days or so."
Does it have to be all the computers?  If you reboot half of them would the problem go away?  Or the other half?

Why don't you presume the modem itself is bad?

I have a hunch this is a DNS issue, but need more information to confirm or deny it.

OTS_Tech,

When you type of "internet issues" do you have? If i were you I'd try isolating the issue, when the issue occurs: are all computers not being able to access the web? what about accessing other internal resources, can you ping other computes and the NSA's lan interface? You said yo have a small network,  I'd also try disconnecting when everything when things aren't working and then re-connect one at time.

A few questions:
1) How many network devices were on power protection at the time the power surge? And is the modem on power protection now?
2) Is the switch managed or unmanaged? If managed, have you checked for abnormal traffic patterns on any given port? You could have a system that's acting strange.
3) When the internet goes down, does internal communication go down as well?

I don't know exactly how small your network is, but you could disconnect devices one at a time and see what happens after 20 mins. Possible that a NIC got impacted from the surge.

Paul McDonald:
1) All traffic is stopped.  They have software on the server that is unavailable as well.  I did not try to ping the server from outside the network but pinging it from inside the network finds it unavailable. Also note that not all the computers are affected at the same time and some actually have the network icon as if the cable is unplugged while others have internet.
2) There are 4 computers and the server, the rest are POS systems and the gas pumps (it's a gas station).  If the systems are rebooted one at a time, there isn't any pattern to what reboot reboot resolves the issue as cycling power on the switch is what clears it out.
3) The modem was replaced the morning after the power issue.

Jorge Diaz:
I think I answered a lot of your followup questions above.  We have brought them up one at a time and it will work for a few days before the drop of all connectivity happens again.  It doesn't happen at the same time for everything and it doesn't happen across the board or in a pattern.

mansrock:
1) Everything is on APC UPS battery backup.  Everything is still on it.  Nobody was there when it happened, when the client came in to find the issue they called their ISP who came out and replaced the modem and rebooted the network devices.  It ran for about 15 minutes before all connectivity dropped again.  Rebooting the switch restored everything again for about 15 minutes.  This is their cycle unless they reboot all the computers and the switch, then they get a few days.
2) It's an unmanaged switch. Initially we thought we had the system isolated so a colleague of mine replaced the NIC card, but the issue reoccurred within a few days.
3) Yes, all connectivity is lost.

I think I've answered everyone's follow up questions, let me know if you need more clarification on anything.  Note, their NSA was already slated to be replaced and is scheduled for Wednesday morning.

Thank you all,
Beth

Have you checked to make sure that everything is connected properly in terms of wiring? Something could be unexpected misconfigured and causing packet storms.

"The modem was replaced the morning after the power issue."
This problem started when the modem was replaced, right?  I was asking if it had been replaced again, but perhaps I misunderstand.  I realize it's not the modem you're rebooting to fix the issue, but that doesn't mean the problem isn't in the modem.

Are your clients DHCP clients, or do they have static IP addresses?  
  If they're DHCP clients, is your server a DHCP server, or do you get IP addresses from somewhere else (like the modem)?
  If they're DHCP clients, do the failing clients have good IP address information assigned to them?

Paul brings up a good point: Is the modem truly a modem, or is a router? And what purpose(s) is it serving? Is there a firewall behind it handling most things in your network?

The modem was replaced the morning the issue first began.  The modem connects to a switch that feeds two networks, the camera network is fine, the data network has the issues. The data side goes from the switch split into a router that is their firewall, then into another switch that feeds the network.  It's the switch between the NSA and network systems that the reboot fixes temporarily.  They do have a server that hands DNS and DHCP.  When the systems are isolated from the network and internet, they do have a legitimate IP within the network.  I did also try to statically set the one system but it still has no connectivity to the internet or network.

Is there any other information I can provide?  I'm not great with diagramming digitally but i can draft you one and send a picture if you think that will be helpful.

If you still have to solve that issue, my approach would be to "divide and conquer" - in extreme, use a switch per single device, just to find out which one is causing the failure. You can also go and just split the data network between two switches first, then split the offending branch aso.

PC/servers are on the same "subnet"?  and you are not able to ping between each other?

Are you able to ping your default gateway ?
or in CMD use "arp -a" to see if you have ARP to the router (default gateway) when the problem accours.
If Cameras works and are on switches as Compueters/PC its a routeing issue happening on the router/firewall.
If you are able to ping Default gateway (router) and not 8.8.8.8 or something else outside i will say  its a modem issue.

Or if you have a dual router with backupconfiguration (vrrp/hsrp or something). it might be a MAC flapping issue or mac/arp issue that makes the network unable to forward traffic as it should do.