More wild guesses... OK then: Looking into my crystal ball I see you're using an ATI card. Unlike other distros FC5 attempts to use your vid card for special effects on the desktop by default. So, despite your deduction that it couldn't possibly be hardware, your vid card may, in fact, be overheating. Perhaps it was a cooler day when the system stayed up 2 hours than the day it crashed in 20 minutes.
http://en.wikipedia.org/wiki/AIGLX
http://fedoraproject.org/wiki/RenderingProject/aiglx
Quoting the last link:
There are three interesting pieces of technology here:
1. An updated Mesa that includes accelerated indirect rendering. This allows the compositing manager to do GL operations through the server instead of having to push window pixmaps from the COMPOSITE extension down to the client and then back up to the card.
2. A version of metacity with a compositing manager enabled. It has a few effects implemented, including window minimzation and drop shadows.
3. An updated X server. This X server is a devlopement version of Xorg that contains support for a new extension to improve performance. It can be installed along side your usual X server.
We are planning on shipping the first two with FC5. They
should function normally in standard use.
