Try this test -> Remove the case cover and see if you get more minutes. If so, heat can be the suspect. Reasons abound from a failed CPU fan to some heat sensitive component.
In closing, be sure no Parasites are on the machine. Read http://www.doxdesk.com/parasite/
Am i missing something obvious or is it more serious. My friends pc crashes after browsing on the internet after 15 minutes yet if you just connect and don't browse - no problems - doesn't crash at all then.
He has had his hard drive formatted and windows reinstalled, a new modem fitted and the problem is still there.
When it crashes it's not a blue screen but multi coloured???
any ideas anyone please