Weird network problem: Ethernet link going down for few seconds, and up again

Question

Hi,
this problem is driving me nuts for weeks now, and I've not had any success with solving it myself, so I hope someone might have a clue...
A few weeks ago, maybe 2 months, when downloading VOD content from iTunes, the download suddeny stopped after a while, saying that there is a problem with my network connection. I can then resume the download until the next connection drop occurs. iTunes was and still is the only app that makes the problem noticeable (FTP, browsing, Mac App Store etc. are working fine), however, as this also affects iDevice firmware downloads and basically everything I download via iTunes, the problem started to really annoy me. I'm not sure, but the problem might have been introduced after upgrading from Lion to Mountain Lion/10.8, which I needed for Logic X.
So here is what happens:
I have an iMac12,2, Mid 2011, 27inch, i5 2.7GHz, 16GB RAM, 256GB Apple SSD + 1TB HDD. When using ethernet, the iMac randomly loses its link to the network. My router is a FRITZ!Box 7390 (german make). There are no switches between my iMac and the router. When that happens while running ping, this is how it looks like:
64 bytes from 192.168.1.1: icmp_seq=3558 ttl=64 time=0.542 ms
64 bytes from 192.168.1.1: icmp_seq=3559 ttl=64 time=0.424 ms
ping: sendto: No route to host
Request timeout for icmp_seq 3560
ping: sendto: No route to host
Request timeout for icmp_seq 3561
ping: sendto: No route to host
Request timeout for icmp_seq 3562
ping: sendto: No route to host
Request timeout for icmp_seq 3563
ping: sendto: No route to host
Request timeout for icmp_seq 3564
Request timeout for icmp_seq 3565
64 bytes from 192.168.1.1: icmp_seq=3566 ttl=64 time=0.508 ms
64 bytes from 192.168.1.1: icmp_seq=3567 ttl=64 time=0.493 ms
And this is what the logfile contains at the same time:
28.10.13 21:36:35,000 kernel[0]: AppleBCM5701Ethernet [en4]: Link down (womp disabled, proxy idle)
28.10.13 21:36:37,099 configd[18]: network changed: v4(en4-:192.168.1.188) DNS- Proxy-
28.10.13 21:36:37,106 configd[18]: network changed: Proxy
28.10.13 21:36:37,112 netbiosd[12000]: network_reachability_changed : network is not reachable, netbiosd is shutting down
28.10.13 21:36:40,000 kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link up on en4, 1-Gigabit, Full-duplex, Symmetric flow-control, Debug [796d,0323,0de1,0300,cde1,3800]
28.10.13 21:36:40,237 configd[18]: network changed: Proxy
28.10.13 21:36:42,544 configd[18]: network changed: v4(en4+:192.168.1.188) DNS+ Proxy+ SMB
So the ethernet interface loses the link for a few seconds, and gets back up again. It's not long enough to affect average TCP connections, however it is long enough to completely screw iTunes.
What I've tried:
- changed all cables (save the one that's going from the ethernet outlet on my room's wall to the server/router room, as it's not replaceable (running within a wall) and I really don't know why it should become faulty offhandedly)
- cycled ethernet ports at the router to make sure they're not faulty
- ping'd the router from different computers on all its ethernet ports, all working fine
- connected my iMac to router via wifi, all working fine
- bought a Thunderbolt Ethernet adapter to make sure onboard ethernet card isn't faulty: problem still there
- installed 10.9 over 10.8.5, problem still there
What I didn't check is what happens when ping'ing the router via my cables, but from another computer, as I don't have one. I will try to borrow a MacBook and check that.
So I figured it must be a local problem, as the network infrastructure itself works nicely and all cables seem to be sound. It is also not a hardware issue, as the problem occurs both with the onboard NIC and the Thunderbolt NIC. What mystifies me is that it doesn't happen when using Wi-Fi (but switching to that is not an option - it's too slow).
I tried to use both DHCP and fixed IP addresses, different IP ranges, made sure that static IPs don't collide with the router's DHCP range, updated the router firmware, updated the firmware of all Airport Express and Time Capsules I've got in my network (just to be sure), switched all energy saving settings to "Never" (as it has been suggested here from time to time), but nothing helped ... and I don't know what else to check.
I hope that someone has an idea of what's happening here!
Thanks in advance!

Answer 1

It looks like hooking an ethernet switch between my iMac and the router did the trick. I didn't get any timeouts after I did that. It must have been a really weird incompatibility between my iMac and the router. Still have to see if iTunes gets through without losing the connection, I hope I'll be able to check that tomorrow night. I'll report back.
So maybe try just hooking another ethernet switch between your mac and the other side. It might actually help!