Random restart

Dell Studio XPS laptop, Windows 7 has mysteriously started rebooting after a random number of minutes, ranging from a couple to 10 or 15. It seems to be related to being on an Internet browser, and it doesn’t matter whether it’s Firefox or Internet Explorer. We’re trying Chrome now. At first, we thought it had to do with being connected wirelessly, but it just repeated the feat while having an Ethernet connection (was okay on that for hours yesterday).