git repo being corrupted?

I've recently started to notice the following being reported:

% git-status
error: bad index file sha1 signature
fatal: index file corrupt

(a) Is there anything I can do to fix this (apart from git-clone from
the repo and start over) ? git-fsck reports the same problem.

(b) What is causing it? Is it just my hard disk failing, or is it a bug?
A memory test seems to be OK.

Linux 2.6.22-3-amd64
File system is XFS
git version 1.5.4.4