Another idea. I have had this problem off and on for years. It depends on Airport being on the network. On a lark I went to System Preferences -> Netwrork, selected Ethernet and clicked on Advanced ... on the lower right. There I selected Ethernet, and set the following: Configure Manually, Speed 100baseTX, full-duplex, MTU Standard. The essential change was to switch from "Full-duplex with flow control" to "full-duplex". Clicked OK, then Applied. No problems since but for a short time. Forgive me if this is premature, but I wanted to log this before I forgot what I did. Any follow up reports would be appreciated.