Fedora on an AMD Athlon 64 (3200+) machine

[Alan]

> I ran memtest86 from the first CD, as recommended,
> but it only got 50% of the way through
> (during test 4 or 5, I think)

If memtest86 is crashing then you almost certainly have a hardware
problem. It might not be the ram but memtest86 generates continual memory
accessess so also generates lots of heat.

memtest86 is a very reliable diagonostic of problem boxes.