Sounds like an IP conflict or corrupted ARP cache. If there are several machines hooked up to the same switch, are you positive they all have unique IPs and their netmask is correct?
If you are using several machines, are you using rfc 1918 private IP range (10.10, 192.168, etc) for each machine?
If the switch itself is acting as a DHCP server, I would check carefully that this is configured correctly. You may want to reset the switch to factory defaults before proceeding. But unless I am mistaken, you are saying that you see this behaviour with two *different* switches entirely, so I would look to the client configurations before I started mucking with the switch.
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