Yesterday (Friday, September 21, 2012), my Internet connection started acting up. After some testing, I confirmed a very specific and baffling set of symptoms:

• Internet connection goes away every 25-35 minutes (I did not confirm the exact interval, but it seems to be about 30 mins.)
• Only some protocols are affected; HTTP*, P2P, etc. stop working; FTP, etc. continue to work
• When it’s stopped, cannot even ping router or cable-modem IPs or view their firmware pages
• Domain-names and IPs are irrelevant (for protocols that stop working, neither work, for those that still work, both work)
• Resetting router fixes it for another 30 minutes
• Keeping the connection idle or active doesn’t seem to make a difference (nor the bandwidth usage in that period)
• Connecting directly to cable-modem allows it to work indefinitely
• Disconnecting the router from the cable-modem works indefinitely (no Internet connection obviously, but can still access router IP and firmware page)
• Connecting the router to the cable-modem, but putting the modem on standby also works indefinitely
• Same problem with both a wireless laptop and wired (on any port) desktop (both Windows 7; will try to test Windows XP when possible)
• Nothing had changed in the days leading up to the issue. No modifications to the networking configuration or the router; there were not even any Windows updates except for an MSSE definition update.
• Waiting does not fix it, nor does any amount of fiddling with anything; only resetting the router fixes it for 30 minutes (resetting the cable-modem doesn't work either)

I tried cleaning the pins in the router’s plugs, but that didn’t help, which was not really a surprise since I was not getting a lost connection error.

Obviously my first thought was that the router was having a problem, and this is borne out by some tests. The problem is that when it drops, it is not a full drop since I can still do things like ftp ftp.mcafee.com and such which means that the connection and DNS are still working. Moreover, if it were the router, then why does it stay alive indefinitely when not connected to the cable-modem (i.e., no outside influence)?

The problem doesn't seem to be either the cable-modem nor the router, but rather an interaction between the two, like something from the outside (port scan? hacker? ISP?) that is triggering a problem in the router. I see that there have been a couple of vulnerabilities for the DI-524, but those were a while back and should be fixed since I have the last firmware for it.

I don’t think it’s my ISP (Rogers) since I have been using the router for several years without problem and can connect indefinitely when bypassing it. But I can’t rule them out since that is one of the only possible things that could have suddenly changed.

Does anybody have any ideas of explanations, fixed, or tests?

(I note that when I opened the router, I heard a very high-pitched noise from somewhere near the capacitors/ferrite ring which I don’t think I heard the last time I opened it a few years ago, but then if it were that, then why would it affect only a very small, specific set of functions?)