Intermittent Ethernet on Optiplex 9020 under Xubuntu 15.04

I have a new Optiplex 9020 in the mini-tower case, with i7-4790 and 32 GB RAM. It runs fine with 64-bit Microsoft Windows 7 Enterprise. I installed 64-bit Xubuntu 15.04 (Ubuntu with the Xfce desktop) in dual boot. That seems to work OK too, with one exception: the wired Ethernet works properly in Windows but is intermittent in Linux.

Specifically, lspci recognizes the Ethernet hardware as an Intel I217-LM. Running ifconfig seems to show all the right values, except that there is no IP address. Under Windows, the device is able to use DHCP to obtain an IP address, but not under Linux. I was able to get connectivity temporarily by using a USB-to-Ethernet adapter, so I have all the latest updates. I tried setting the IP address, mask, and gateway manually, to match what DHCP provides, but I still get the message that all destinations are unreachable.

I tried un-installing the standard driver and re-installing it, but that didn't help. Then I downloaded the latest driver from Intel. After several tries, DHCP started working. This survived several reboots, so I thought the problem had been solved. But a few days ago it stopped working again.

I did try booting from CD using various live Linux distributions. Ubuntu and Fedora had the same problem, but Knoppix recognized the port and worked properly with DHCP.

I called Dell tech support, and they said that even though the Optiplex 9020 is certified as compatible with Ubuntu (version 12.04), since I had not bought it from Dell with Linux installed they could not support me. So I am hoping that somebody out there has some ideas.