CPU context corrupt?

[Aquila]

Hi

I have had this problem quite a while now: when playing tribes2 or when
a particular CPU intensive xscreensaver is running, X would often hang.
I used to be able to ssh from another box or use SysRq-K to kill X and
restart (but I never figured out what the problem was).

Ever since upgrading to 2.4.9-ac3 (from 2.4.8-ac5 I believe), whenever
it hangs in X the computer would beep and give this message in syslog:

Sep  4 21:43:41 hamlet kernel: CPU 0: Machine Check Exception:
0000000000000004
Sep  4 21:43:41 hamlet kernel: Bank 1: f600200000000152 at
7600200000000152
Sep  4 21:43:41 hamlet kernel: Bank 2: d40040000000017a at
540040000000017a
Sep  4 21:43:41 hamlet kernel: Kernel panic: CPU context corrupt

It hangs there, and SysRq-K is no longer able to kill X properly. sshd
stops working as well. What does this message mean? Do I have faulty
hardware? 

My CPU is an Athlon 1.2Ghz, not overclocked. Any ideas what's wrong?

Thanks,
Aq.