Jeff:
Have you considered the possibility of an environmental problem? Assuming it is a desktop PC with a metal tower case, if you put your hand on the rear of the case near the top, does it feel very warm/hot to the touch? If so, I would power off, open u the case and turn it back on. Then check every fan to make sure they're all running. After that, power the system off and watch each fan spin down. There should be a 'coast-time' for each fan. Any fan that stops abruptly should be considered suspect. Be particularly concerned about the processor fan and the power supply fan. I'm sorry I can't be more specific but I was unable to find the model you described on Dell's website. Dell usually lists their models as Dell Dimension or Dell Poweredge or some other word followed by a string of letters and/or numbers. Anyway, please post it if you can find it.
You may still have a networking problem even if you NIC passes all tests. The best test for wiring is an end-to-end test with a LAN tester. In the absence of such equipment, at least check for improperly terminated horizontal cabling (in the walls) and replace the patch cables affecting this PC with others known to be good.
Try moving the PC to see if it fails using a different drop of horizontal cabling. Try plugging into a different port on the switching hub (or better yet, a different switch). Check the switch to see if it feels excessively hot.
To summarize, software (usually) doesn't fail intermittently. Focus on a hardware problem. Please post details of your activities and the results so we can continue to offer help.
HTH
Roger
Make a great day!
Roger
