The latest Airport and TC suffer from some WAN port issues.. If you have self assigned IP the router has stopped passing addresses but it is clearly more serious .. it has stopped routing.
You need to do a test totally without the Airport router..
So plug the Mac into the (fibre ONT or Router??) and do your torrents.. does it maintain the connection and finish the download??
Test for a long enough period you can be certain the AE is the fault. (or NOT). Sadly though the AE is often the cause of this.
Switching the Airport from "Bridge Mode to DCHP" neither, removing Keychain password... nope. Restarting the MacBook... Nothing at all.
When you switch to bridge.. try again.. this time plug the main router to a LAN port instead of WAN..
If you still get the issue.. you are actually seeing a bug in the Airport.. you can replace it with a new one.. but it will be unlikely to fix it.
Yosemite is not a help.. It is the poorest of the poor releases with network bugs as long as your arm.
Here is my standard instructions for setup of Airport / TC with Yosemite.. but also note.. i think you have something more fundamental going on with AE here.
Please read AE for TC.. they are the same thing.
A factory reset of the TC is the start.. it is required so you can find it again.
Factory reset universal
Power off the TC.. ie pull the power cord or power off at the wall.. wait 10sec.. hold in the reset button.. be gentle.. power on again still holding in reset.. and keep holding it in for another 10sec. You may need some help as it is hard to both hold in reset and apply power. It will show success by rapidly blinking the front led. Release the reset.. and wait a couple of min for the TC to reset and come back with factory settings. If the front LED doesn’t blink rapidly you missed it and simply try again. The reset is fairly fragile in these.. press it so you feel it just click and no more.. I have seen people bend the lever or even break it. I use a toothpick as tool.
N.B. None of your files on the hard disk of the TC are deleted.. this simply clears out the router settings of the TC.
Setup the TC again.
ie Start from a factory reset. No files are lost on the hard disk doing this.
Then redo the setup from the computer with Yosemite.
1. Use very short names.. NOT APPLE RECOMMENDED names. No spaces and pure alphanumerics.
eg TCgen5 and TCwifi for basestation and wireless respectively.
Even better if the issue is more wireless use TC24ghz and TC5ghz with fixed channels as this also seems to help stop the nonsense. But this can be tried in the second round.
2. Use all passwords that also comply but can be a bit longer. ie 8-20 characters mixed case and numbers.. no non-alphanumerics.
3. Ensure the TC always takes the same IP address.. you will need to do this on the main router using dhcp reservation.. or a bit more complex setup using static IP in the TC. But this is important.. having IP drift all over the place when Yosemite cannot remember its own name for 5 min after a reboot makes for poor networking. If the TC is main router it will not be an issue.
4. Check your share name on the computer is not changing.. make sure it also complies with the above.. short no spaces and pure alphanumeric.. but this change will mess up your TM backup.. so be prepared to do a new full backup. Sorry.. keep this one for second round if you want to avoid a new backup.
5. Mount the TC disk in the computer manually.
In Finder, Go, Connect to server from the top menu,
Type in SMB://192.168.0.254 (or whatever the TC ip is which you have now made static. As a router by default it is 10.0.1.1 and I encourage people to stick with that unless you know what you are doing).
You can use name.. SMB://TCgen5.local where you replace TCgen5 with your TC name.. local is the default domain of the TC and doesn't change.
However names are not so easy as IP address.. nor as reliable. At least not in Yosemite they aren't. The domain can also be an issue if you are not plugged or wireless directly to the TC.
6. Make sure IPv6 is set to link-local only in the computer. For example wireless open the network preferences, wireless and advanced / TCP/IP.. and fix the IPv6. to link-local only.
There is a lot more jiggery pokery you can try but the above is a good start.. if you find it still unreliable.. don't be surprised.
You might need to do some more work on the laptop itself. eg Reset the PRAM.. has helped some people. Clean install of the OS is also helpful if you upgrade installed.
Tell us how you go.
Someone posted a solution.. See this thread.
Macbook can't find Time Capsule anymore
Start from the bottom and work up.. I have a list of good network practice changes but I have avoided Yosemites bug heaven.
This user has had success and a few others as well.
RáNdÓm GéÉzÁ
Yosemite has serious DNS bug in the networking application.. here is the lets say more arcane method of fixing it by doing a network transplant from mavericks.
http://arstechnica.com/apple/2015/01/why-dns-in-os-x-10-10-is-broken-and-what-yo u-can-do-to-fix-it/
No luck replace the AE with a different brand.. fibre connections (I think your speed is only possible with fibre) are often problematic with Airport routers. They seem to work best on US type cable connections.. but testing on fibre is limited and I have no luck with ADSL.