Server networking suddenly flaky

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http://www.linuxquestions.org – 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 d (HowTos)