Having very similar issues except when the speed on mine drops to zero, my computer can't access local network or the internet. However all other devices on the network remain alive. Outlined in detail below:
The Issue: When using download accelerator/managers such as Down Them All or Turbo Download Manager, connection is intermittent (upon reaching peak speed of anywhere between 6-14MBps) and then drops to zero, resumes, then speed climbs up and down like this until the download is finished. Internet / network is inaccessible via this computer only, when speed drops to zero. Networking / wifi / internet remains in tact and can be accessed and used by other devices while the local machine is having this issue.
When did it start?: When I recently introduced 802.11AC routers into my home network. I’ve encountered this issue on three different models / brands. (Netgear R6400 returned after experiencing this issue + Netgear R6250 returned after having same issue as the first and most recently I have installed an Apple Airport Extreme current version, which seemed the most stable at first and is now showcasing the same intermittent speed drops). Was not happening at all with my Netgear N900 WNDR4500 Dual Band Wireless N router.
Setup: The computer having this isolated issue is a Mid-2010 iMac 27” 2.7ghz Quad Core i5. Issue is not found on my late 2014 Mac Mini, which is equipped with 802.11AC wireless. Modem is a Motorola SBG6580 with Nat turned off so that it isn’t acting as a router also. Router, as stated before, is Apple Airport Express, current version, with a Mac Mini and WD MyCloud wired in. There are also 2 apple tvs, a Samsung Tablet and 2 iPhones that connect wirelessly on and off to the system without issue.
What I’ve tried to do: Thinking at first that this was some type of incompatibility issue between the newer wireless AC routers and the wireless N card in my IMac, I went about switching routers after trying a number of troubleshooting measures.
-Changed my DNS to Google 8.8.8.8 and 8.8.4.4
-Lowered Fragmentation and Threshold values to recommended values by Netgear (Which dropped my download speeds to less than half, increased avg ping time to 200+ms vs 15-20ms, prior to change) seemed to stabilize the connection without dropping, but makes the whole upgrading to an AC router Nil, if performance is limited that severely.
-Changed MTU from 1500 (to 1453 and 1400 respectively)
-Deleted various wifi related plists in systemprefs in OS X.
-setup a new wifi from network preferences
-removed the whole SystemConfiguration directory
-killed discoveryd
Also tried changing number of segments per download and loading last kilobytes first in Down Them All Manager preferences. Again, running the same process with identical downloads and software on my AC equipped Mac Mini, I am unable to replicate the glitch, both wired or wireless.
Why do I think the issue is in Firefox: Upon figuring out that, if I quit firefox once I notice the speed start to drop after reaching peak download speed (thusly canceling the downloads in the Download Manager/accelerator) the connection stays alive. I am able to use the internet just fine in another browser. If I let the download continue until it drops to zero, the connection is dead momentarily…unable to use local network or internet for the duration of it being at zero. Again it will resume automatically (anywhere between a few seconds to up to a minute+ in some case) and the wifi connection itself is never actually disconnected. If I close before it drops to zero and I promptly reopen the window, the download will resume and the cycle will continue. This never happened when using my wireless N router, because perhaps it was never able to achieve the speed/throughput that these AC routers are putting out, which thusly maybe overloading my system? That’s the only layman explanation I can think of (I admittedly do not know any networking speak or the technical explanation beyond what I’ve explained), either that, or my network card (802.11n) simply can’t maintain a steady connection at peak speed with these 802.11ac routers. Over the 802.11ac, I can even do high speed file transfers over my local network consistently without a hiccup, so my only hunch left (save for going out and buying a new AC Wireless adapter) is that something in the way firefox and these download accelerators are operating are causing it to get messed up in a way that my computer cannot handle, despite it not being a lightweight in the performance dept. PLEASE HELP!
