Hi R,
We've not seen this happen to others.
What version of the OS is this?
As you suspect, I also think it's likely a hardware issue.
The MacOS memory test is known to often "pass" bad memory.
Having said that, you could first try running the memory test in single-user
mode for a little more thorough RAM test. If it still doesn't catch an issue,
you may want to try a third-party memory testing app, such as "memtest"
http://www.memtestosx.org/ (I just checked, but they don't have a OSX 10.6
version, though).
Let us know how these tests go.
-shinya.