Bizarre and very specific Internet connection loss

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)
• If I keep pressing the standby button on the cablemodem after browsing some pages, the connection works indefinitely (If I press the button to restore the connection, browse some pages, press the button again to cut the connection, it works fine for hours and hours without issue. Only when I leave it active for a while does it finally stop. I repeated this for three days with the same results.)

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?)

Update

Last week, my Internet connection dropped out for two days. I called the ISP and they had me bring the cable-modem in to be replaced. I exchanged the SB5100 for a new cable-modem that has a built-in router with wireless AP and firewall. (I left it in gateway mode instead of bridge mode.)

This morning it started behaving just like the old router. Most protocols continued to work (mail, P2P, NTP, etc.) but web-browsing (HTTP) was dead; I could not even connect to the cable-router’s web-interface. I rebooted the cable-router and HTTP functionality was restored; exactly like the previous router.

I find it highly unlikely that this could be a coincidence or related to the router itself since the new one is literally brand-spanking-new (bagged, twist-ties, in a box, with that new-plastic smell, etc.), and completely different from the old one (the new one is an ISP-branded model labeled “Hitron” while the old one was a D-Link DI-524).