While running, laptop will reboot and the...

I have a Lenovo laptop (3000 V100) with a swapped-out 250 GB HD, which I love. I’ve only had it three weeks. But I have an intermittent problem (about twice a week). I will leave it on while, say, eating lunch, and come back to my office, and find the screen black, with the message "PXE-E61: Media test failure, check cable" and "PXE-M0F: Exiting PXE ROM" "No OS found." "Insert OS setup disk, then press any key." Pressing a key will just yield the same message. If I turn it off and back on, it boots to XP without any hesitation. It happened once shortly after I had booted up and was running a single program, and one other time my computer seemed to freeze up while running several applications (spreadsheet, word processor, Juno email), so I hit "Crt/Alt/Del" to bring up task manager. It rebooted instead, and went to the above "PXE" screen. But all other times, it has happened when I’m just out of the room for a bit, leave the laptop on, and return to the "PXE" screen. Again, it has always rebooted smoothly. The seller (still under warranty) had it for a few days, reseated the drive, loaded and unloaded data, but could not get the thing to crash. He recommended I replace the after-market HD with the original 100GB one and see if it crashes with that. Unfortunately, we now cannot locate the original HD (it's around here somewhere!). I see several forum messages on the web that address the error message, but most folks seem to see it on initial boot up. I have never (knock on wood) had initial boot problems - just mid-use. So I may have two problems, one that causes it to reboot suddenly, and then a boot problem under that particular circumstance. One suggestion was a bad ethernet connection of some kind, but I use a wireless connection to internet, no cable. Any ideas? It otherwise performs very well.