Q: Intermittent wi-fi power loss (?)

System details: Macbook Pro, 15-inch, mid-2012. Processor: 2.6 GHz Intel Core i7. Memory: 8 GB 1600 MHz DDR3. Running OS X Mavericks 10.9.5.

Wireless network details: Router is an older Time Capsule. Firmware is up-to-date. Security: WPA/WPA2 Personal. 802.11b/g/n. My ISP is Time Warner Cable.

Symptoms: Since September 29, 2014, while I'm working, the machine will intermittently lose wifi connectivity. When this happens, clicking on the wifi status icon in the menu bar shows that wifi is on, but no networks are listed. (Other wifi-enabled devices -- my iPhone 5 and my husband's Macbook Pro, for example -- can see and connect to the wifi network just fine at the time when this happens.) When I click on Apple menu - System Preferences - Networks, then choose Wi-Fi from the list, it says Status: Off. (This, despite the fact that the status in the menu bar still says On. I have screenshots of these conflicting statuses.) When I then click the "Turn Wi-Fi On" button, the status in the menu bar changes to Off, but the status in the System Preferences - Network panel remains Off. When I click "Turn Wi-Fi On" again, the button depresses and turns blue briefly (showing that it registered the click), but nothing happens to the wifi status in either the menu bar or the System Preferences - Network panel --- the status remains Off.

In short, when this happens, the wi-fi acts as though it is turned off, and I lose the ability to toggle wi-fi power.

Restarting the machine usually fixes it. After restart, wifi is On and connects properly to the network. However, after a period of time anywhere from one minute to 24 hours (that I have observed so far), the symptoms described above recur and wi-fi connectivity drops again. I have been keeping a written log of this behavior for a few days, recording the times when it drops and the times when it is working properly again after a restart, and can ascertain no particular pattern to the length of time between drops.

Occasionally -- twice out of probably 15-20 restarts so far -- upon restart, wifi connectivity is not restored. Instead, the wifi status icon in the menu bar is gray with an X in it, and clicking on it elicits the message "Wi-Fi: No Hardware Installed." If I then restart the machine again, wifi will work properly (for an unpredictable period of time, as described above).

I have run Wireless Diagnostics and had it monitoring the wireless connection several times when connectivity dropped. It noticed the drop all but once and notified me, and I output the log files. Usually, after writing the log files, it will tell me that wifi is turned off and ask me to turn it on. However, one time it told me that there was a WAN failure -- that my wifi was working correctly but it could not connect to the internet. Also, as I suggested, there was one time that connectivity dropped with the same symptoms as above, but Wireless Diagnostics did not notice that it had dropped.

In my written log, I have also been tracking what I was doing and the physical position of the machine when connectivity drops. A few times, wi-fi connectivity has dropped immediately upon setting the machine down in another place (when I picked it up and moved it while it was open and awake). However, many more of the drops have occurred when the machine was sitting still in one place and I was working. Also, I logged at least two instances where I moved the machine and set it down and wi-fi connectivity did not drop. The drops that have occurred when the machine is moved are equally likely to happen whether it is being moved farther away from the router or closer to the router.

Physical proximity to the router (and thus stronger signal) does not appear to have any correlation to whether wi-fi connectivity drops, or on the length of time between drops. Physical position of the machine -- on a desk, on a stand, on my lap -- also does not appear to have any correlation.

What I am doing on the machine also does not appear to have any correlation to the problem. I could be composing an email (typing quickly), browsing the web (occasional clicking, much scrolling), or just standing there staring at the screen and not touching the machine at all.

When I connect the machine directly to the cable modem using an ethernet cable, that connection works just fine. This is a wifi-only issue.

The very first time I observed this behavior, I had brought my machine into a coffee shop and had just opened it up and tried to connect to their wifi. Immediately beforehand, I had been at a university library, connected to their guest wifi network with no problems (so it's not like it dropped at home and I didn't notice it until I opened up the machine at the coffee shop). This would seem to imply that it doesn't just happen on my home network, and is therefore probably not caused by my router. However, I haven't directly tested it with a different router.

Putting the machine to sleep and then waking it does not trigger the symptoms.

Things I have tried:

• Restarting the machine, obviously. This restores wifi connectivity temporarily (except for the occasional time when "Wi-Fi: No Hardware Installed" is shown).
• Unplugging the router, waiting 60 seconds, and then plugging it back in.
• Unplugging the cable modem, waiting 60 seconds, and then plugging it back in.
• Resetting the PRAM. No effect.
• Resetting the SMC. No effect.
• Going through the troubleshooting steps at Wi-Fi: How to troubleshoot Wi-Fi connectivity.
• The Genius Bar installed a new Airport card. Note that they were unable to reproduce these symptoms while they had the machine in the store for about 48 hours. Their extended diagnostics showed no hardware problems, either with the Airport card or with any other piece of hardware in the machine. They replaced the Airport card based on my description and screenshots of the symptoms, especially the "Wi-Fi: No Hardware Installed" message. The problem recurred a few hours after I got the machine back home.
• Turning off Bluetooth. No effect.
• Running a scan in Wireless Diagnostics - Utilities window - Wi-Fi Scan (while the wi-fi was working properly), and changing the channel on the router to one of the channels that Wireless Diagnostics labeled "Best." I did this about 4 hours into the 24-hour uptime interval. I really thought that had fixed it, because 24 hours was by far the longest it had stayed up, but then it dropped again.
• Erase and install. I did this two days after the Airport card was replaced. I followed the instructions at OS X: How to erase and install to erase the hard drive and install Lion from the recovery partition (because that's what was on there), then re-downloaded Mavericks from the App Store and installed it. I had the machine connected to the internet via ethernet cable during this process. I did not reinstall any apps or restore any of my files from my backups, in case one of those was the problem -- left it as a completely fresh install. The problem still recurred within a few hours.

I have a whole bunch of Wireless Diagnostics logs as .tar.gz files. I've untarred/unzipped a few of them and tried to look at the logs myself, but I can't interpret them very well. I can tell that Wireless Diagnostics keeps testing to see whether the connection is working, and I can see when Wireless Diagnostics notices that it's not working, but I can't see anything obvious that explains why it's not working.