I've had a server running for over a year and it's been very stable. Suddenly (no config or software changes) it's behaving oddly. If I perform a network related activity, it appears to effect other separate network functions. Example: I get onto the machine using realvnc server and suddenly port 80 goes down, or I can access port 443 inside the network but not externally, or I can get to 443 but not 80. The server has two network cards. Switching to the second card appears to have had no effect. I also tried rebooting, switching in a new network cable, bouncing the network card (which does re-establish services, but not consistently). I have several servers and this is the only one misbehaving. It's running Debian Etch (as are a few others). Any thoughts would be appreciated. In all other ways, the server is functioning properly.

Sounds like there's a problem further upstream. You don't say whether or not those other boxes are on the same switch, but it could be the port on the switch/router is going bad.

If you've changed no hardware, have tried a different NIC, and not changed any software, that doesn't leave alot of other places to look.

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Thanks for the response TBOne.
I had already tried changing the port on the switch. I'd think firewall except it's also flaky inside the firewall. Some machines can get to it, others cannot. I'm reducing it's Postfix agents ans external database connections, perhaps it's running out of tcp connections.

Good catch...didn't think of that, until you said it.

Where is the thanks button? I reduced connections and it's still not stable.

My bad, another machine being set up was mistakenly assigned the same IP

Not your bad at all...hell, if I had a dollar for every time that happened, well....I'd have at least $20.