James, could you give us some further information about the repository that
triggers this issue?

I presume from what you've said that it's in darcs1 format.  Here's some other
information that would be helpful in trying to reproduce this:

How many patches are in it?

What is the total size of _darcs/patches (e.g. `du -sh _darcs/patches`)?

What's the total size of the repository contents (e.g. du -sh _darcs/pristine)?

How much memory and swap do you have? Similarly, if you watch top, how much
memory do you see darcs using before it dies?

Do you have many (or large) binary files in the repository?

Thanks for your bug report!

David