Invalid page header

I get the following error message when doing a select on a table:

ERROR:  invalid page header in block 295 of relation "reported_titles"

I found some messages that said this means a block of this table is
corrupt.  I found some suspicious lines in the server log just before:

ERROR:  could not access status of transaction 3651584
DETAIL:  could not open file "/usr/local/pgsql/data/pg_clog/0003": No
such file or directory

How do I fix this corruption?  I have dumped as much of the databases as
I can including about half of this table.  Only this table is corrupt.

What could cause the corruption?  We are using custom C code.  Could
bugs in this be causing it?  Or is hardware problems more likely?

  - Ian