The most common reason for this kind of sporadic problem is that the ISP is having issues, and may not even know it until someone complains. It could be your system, and it could be your router, but I think both of those are less likely. You can run a couple of simple tests.
First identify the IP of your router if you don't already know it. Typically it will be 192.168.0.1, or possibly 192.168.1.1. Once you know it, open up a command prompt and ping it several times. You can use the -n x argument to specify the number of pings to send. For example: ping -n 20 192.168.0.1
If an echo doesn't come back (i.e. the packet was lost at the receiver or in transit) then you'll see "Request timed out" for that ping. If you see any of these then either your router has a problem, or there is a problem with the network between you and the router, or there is some basic problem with the IP stack on your system. If the network is wireless there could be interference of some kind, or a weak signal. If it's wired the cable could be bad or damaged. If you're sure it isn't the network, then check the router carefully. If you can confirm the router operates correctly, i.e. by pinging it from another system, then your system would be the only other explanation.
If you get all the pings back without any timeouts, then the issue isn't between you and the router, and at least with respect to the LAN the router is working and doing its job. The next thing to test is packet loss to the WAN gateway. This is the machine your router communicates with at your ISP. To determine the WAN gateway open a browser window and point it at the IP of your router. For example:
http://192.168.0.1
This will bring up the admin page. If you have never accessed it and set the password, then the login is 'admin' and there is no password. For most DLink products the information you want is under the 'Status' tab. Look at the section entitled WAN and write down the IP address in the 'Default Gateway' entry.
Go back to the command prompt and ping the gateway IP twenty times or so. If all you see is "Request timed out" then your router is configured to block returned ICMP packets. This would be unusual, but if you run into it reply and we can talk that. If the problem is what I suspect it is, then you may see a few timeouts. You should also look at the return times for the pings. You'll see "time=XXms" in the returned information from the ping. When I ping my Comcast default gateway I get packets back in way less than 10ms. If you see anything higher than 20-30ms your connection to the gateway is pretty slow, imo. When I had problems during the initial install in our neighborhood I was seeing 70ms to >100ms ping times, and often 50% or better timeouts.
If you see results like mine, then what's happening is that there are problems, probably local to your neighborhood, that are causing packets to be lost between you and the gateway. If all the pings come back, and with good times, there is one thing left to check before getting on the phone with the ISP. Go back to the router admin status page, and look at the entry for DNS under WAN. You should see at least one, and likely two, IP addresses. Write them down. Go back and do the ping test again. If you get packet loss or high latency with either of these addresses then the ISP has a problem with their DNS servers. When I ping Comcast's DNS servers I get all packets back in <14ms.
Any packet loss in the last two tests, or very high latencies, indicates that the ISP has a problem in their network. Call them up and have text captures of the ping results that you can email them.
The last thing to mention is that even if all these tests are successful, the ISP could still have issues elsewhere in their network, or in a network they are peered with. If you know of a site that causes you problems more frequently, then use the tracert command to trace the route to it. For example: tracert
www.anandtech.com
Tracert may or may not work for you, however. A lot of DLink routers (mine included) discard ICMP TTL expired packets, which are used to count the hops between you and the site you are tracing to. In my case I have never been able to get my DI514 to let them through.
Hope this helps, and good luck with it.
