We have four remote sites with PIX 501's.
They are approximately two years old, but we are unaware of how long this issue has persisted.
We have found all of them incredibly sensitive to movement. If you stamp on the floor near one of them, it will reboot.
If you keep a console cable connected, you can see it just looks like a power cycle, as though there was a short or a small power outage.
What gets me is, it seems to affect all of them.
We are finding this difficult to deal with because it brings the VPNs we run offline, and they often take some time to reconnect.