There are a few deeper diagnostics you can perform here to hopefully isolate out whats happening. The easiest one is to leverage the WiFi signal diagnostics within the OS. Clicking on the menu bar WiFi signal while holding the Option key will give you some details on the connection. If you open the network control panel you can also run diagnostics by clicking on the Assist me...' button.
Frankly, I find these very limiting as you can't see something that is fleeting as you have here. This is where good promiscuous mode diagnostic tool can help. Here's what I use: WiFi Explorer as you are seeing all of the signals from all of the local AP's you can see if your system is the one failing or if the given connection to your AP is having problems.
As WiFi is so prevalent we now see AP's sitting on each other which interferes with the signal your system has with the AP you want to hook into. So what happens here is the system needs to re-establish its link using a different channel causing the wink out. What you'll want to see with this tool is what your AP is located on and what channel or band you can switch to. If you see all of the AP's wink out you then know you have a system level issue which is very rare in this series.
If your AP offers dual 2.5 & 5 GHz bands I would start off by renaming them so you can distinguish between them. I would append the bands frequency 2.4 & 5 to the AP's name: i.e. Alpha_2.4 & Alpha_5 This will make things much easier!
Now lets talk about what makes these bands different:
To start with there are still many more 2.4 GHz AP's out there than the newer 5.0 GHz units so getting stepped on when the AP is set to auto is often an issue. But, 2.4 GHz is able to work at a larger distance than the newer 5.0 GHz.
5.0 GHz does have some powerful incentives! To start with it offers faster data throughput than 2.4 GHz and being newer less likely to be stepped on if left to auto. But its weakness can also be its strength as its less likely to be stepped on in denser areas. As the signals won't travel as far.
Lastly, if you have a large house and/or have lots of devices all fighting for bandwidth think about setting up a WiFi mesh. This is were you setup multiple AP devices which ideally are interconnected over an isolated Ethernet network (some use a second WiFi channel which I don't recommend) to your cable router. As the devices are scattered around you can use the 5 GHz band without an issue and get off of the 2.5 GHz band as much as you can (some of your older devices may hold you back).
You keep talking about 'Auto' what is that all about?
Well, within each of these frequencies you have a collection of bands and when the AP device is set to auto it tries to use an unused band. But, the problem with this is devices signals can come in stronger after then have started up this happens when you are linked and walkway from the AP so the AP & your system increase its signal as it doesn't want to loose the connection. So your neighbor who's AP is also sitting on the same band gets walked over as your signal over saturates his.
There are other factors that also play into this but this is the simplest example. Often the bands a device uses is pre-programed into it so all of the same units start at the same band and then work up from there. By using a promiscuous scanner tool you can see what the devices are as well as what bands they are using. I strongly recommend you manually set yours to a band that is less used. If you can, talk with your neighbors to come to an agreement on what their AP's will use. While this won't stop the problem it can at least reduce it. Some apartment/condo & office buildings now have policies on setting up an AP as its become such a problem.
So far we've talked about the WiFi signal side of the issue. As you had already swapped out the AirPort and checked the antenna's I would still look at the signal side of the issue first. Sometimes you need to look at a problem for a length of time to find it. There is just not that much here that can go wrong within the system from a hardware perspective.
There is some software type of issues like not having enough free space on your drive or having some malware or trojan interfering with the OS. Do you have a good antivirus app running and lastly after backing up have you tried reformatting the drive and re-installing the OS and core apps?
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