Intermittent internet/communication outages

Question

     I've been struggling with an issue with my internet connection for years, and I cannot find a solution or even a probable cause for the life of me. Any ideas or suggestions would be appreciated greatly.
     I have a modem connected to my Time Capsule (4th gen) with Cat5e, and an Airport Extreme (1st gen) in a different room to extend the wireless range of the network. It has a printer connected to it with a usb cable. As I use the internet—I think heavy use (large downloads, videos, etc.) trigger the issue more quickly, but I'm not sure—I will eventually become disconnected. I will remain connected to the network, but I will not be able to ping web servers (by domain name or IP address), or even the access points that I am supposedly still receiving a signal from. Disconnecting and reconnecting fixes the problem after a couple seconds of connectivity. Most of the pings will simply time out, but on occasion, I will get a message reading "Reply from [my computer's private IP]: no route to host". I have reproduced this issue on both a mac (OS ~10.7 to 10.9) and a PC (Windows 8.1). Windows is unable to diagnose the problem using its built-in utilities. Left alone, I think the issue eventually fixes itself, but it seems to return eventually anyway. One computer on the network is usually affected at a time; they do not go down simultaneously.
     Changing the firmware from 7.6.4 to 7.5.2 (and to the ones in between, and back to 6.4 again) seems to temporarily help (sometimes), but doesn't really change anything. I have a wireless phone, but it operates outside of WiFi frequencies (I checked). Restarting has appeared to help at first in the past, with the problem taking ~1 hour to resurface, but it now seems to do nothing. A wired connection (i.e. ethernet) seems to solve the problem; but because it is intermittent, is often stalled temporarily by changes, and I wasn't able to test it for very long, I cannot be completely sure. I have analyzed the channels of nearby WiFi networks, and found that no other networks were on channel 1—they were all on either 6 or 11—so I switched it, with little to no effect. I had/have very good reception on both affected machines, with both reading full signal strength (though the strength seems to sometimes weaken to around half on Windows when the problem occurs). When the problem isn't in effect, I get 20mbps down and 5mbps up (according to SpeedOf.Me).
     My internet connection isn't on the verge of collapse, so I don't see why it would do any of this. Further, it's very strange that I can ping neither wireless access point when the issue arises. Tracert (traceroute on OSX) and pathping both confirm that, when I want to talk to either device, I connect to it directly, not through the other one. I have never heard of a network suddenly going silent on a client for no reason, only to communicate again once it reconnects.
     I'm at the end of my rope with this. I've asked about this in other places, and received no replies at all. If you have anything to offer, please ask me about it. Thanks.

Answer 1

Since I don't know what troubleshooting steps you have performed so far other than what you have posted, I would recommend the following general approach in an attempt to find the culprit(s) causing your networking issues.
I would start with your Internet modem. What is its make & model? Is it older than 3 yrs old? If it is, then have your ISP replace it with a newer model. If not, and as a minimum, have your ISP 'test' the line between their service and the modem to verify that the modem is receiving a clean signal.
Once the modem & line are verified, connect a single computer to the modem and use this connection for at least 2-3 days. Any issues with this temporary setup? If not, go on to the next step.
Note: Perform this step ONLY if your modem is a simple modem, not a combination modem/router or gateway device. Perform a 'factory default' reset on your Time Capsule (TC.) This will get the TC back to its "out-of-the-box" configuration. It will default to an Internet router and broadcast an unsecured Wi-Fi network with a network name (aka, SSID) of something like: Apple Network NNNNNN. Once the reset has been completed connect it to the modem using a known good Ethernet cable. Connect the cable between the modem's LAN (or Ethernet) port to the WAN (circle of dots) port on the TC. Power up the TC. Connect a wireless client to the TC and verify that you can access it with the AirPort Utility. Configure the TC as required for a non-extended network. Again use a computer on this network for 2-3 days to see if any issues reappear.
Please post back your results.