Still might be something whack, and the case is the only thing you ~haven't~ eliminatedI hadn't, but I mean, there are no other issues? Ping tests are fine, all my other network based software works?
Last time I ran into this, it was literally something equally strange that we couldn't track down - it would be fine, then not fine, then fine, and so on. It was eventually the chassis - literally the metal box (hell, we replaced the LCD panel on the front of the damned server - twice - before giving in).