Yeah, I should have mentioned. Nothing unusual in the router logs, just the normal DHCP requests/assignments, everything looks fine from there.
I found out it's not actually a problem on the whole router, was just a couple ports, the others were fine. I decided to test the network cables since that'd be an easy one to try, and took one of the cables running off a known-unproblematic port to my laptop that previously had issues. It seemed to work fine that way. So I tried switching the cable over to the other port, and it started having issues again (so, not a cable problem). After messing around some more, I managed to make all the LAN ports stop working, but unscrewed that.
Next, I tried hooking my Xbox 360 up to that same port, with the same cable. No issues, connects immediately and fully. So now I'm thinking it's a NIC issue on my laptop(s), but this is where it gets strange. I boot up my desktop (which is running Win 7 x64, just like one of my laptops, while the other problematic laptop had Win XP x32), and plug that in to the problematic port, and it has the exact same issue. Swap it back to the port it was on before, no problems at all.
So, it seems like my Windows-based computers suddenly, and without any apparent reason (not even a Windows update, I'm 99% sure I changed nothing on any of my devices or my router when this started happening), are having issues with LAN ports 2 and 3, while 1 and 4 work fine, and my one non-Windows device (X360) works flawlessly on all LAN ports.
I don't even know what the hell to make of this bizarro mess. It seems like it must be some kind of router thing, but since it happened suddenly without warning and didn't go away even after a reflash and wipe of NVRAM, I'm not sure what to think. Any suggestions or ideas (besides "hurf durf Windoze Micro$oft lol") would be appreciated, I don't know what to try next.
Edit: Oh, having the devices get their IP from DHCP or static IP makes no difference. I've tried it both ways.
Edit 2: It doesn't look like the device even realizes it's connected to anything during that 10 seconds that the light is blinking slowly on and off. As far as the network adapters list in Windows is concerned, it's not connected to anything (not even trying to connect) until it finally establishes a connection instantly once the LED light comes on and just flashes for traffic like it should.
Edit 3: The cables that I've always used on LAN ports 2 and 3 for the last year or so are significantly longer than the cables on ports 1 and 4. Do longer cables draw more power through the port on the router or anything like that? Could it be making the ports fail faster than others?