debugging kernel panics

[Adam Funk]

I had 3 kernel panics in May on a machine that runs all the time as a
home server and part-time desktop machine.  Each time I found the
computer unresponsive with two of the keyboard lights blinking, & the
syslogs & other evidence suggested that it had panicked once during my
back-up script (which has several combinations of tar piped to gpg) &
twice early in cron.daily (once during aide).

I suspected memory because a few months ago I added some secondhand
memory that a colleague gave me, but I ran Memtest86+ (v4.20) for two
full passes and got these results:

Cache  on
ECC    off
Test   Std
Pass   2
Errors 0

I'd appreciate any further debugging suggestions.

Thanks,
Adam