Missing files under pg_data following power failure

Our testing involves cutting power to machines running postgres 7.4,
while under load (lots of reads and writes). When we do this, we often
lose some files under pg_data storing table content. I.e., the file
named for a table's pg_class.oid value is simply gone. This can affect
many tables following a power outage. We know this problem has
occurred when we are unable to access a table, e.g.

     ERROR: relation "xxx" does not exist

The table is present in the system tables, but the file storing table
content under pg_data is missing.

Can anyone provide insight on this phenomenon? Why are these files
lost? Are they really lost, or have they simply moved somewhere? What
happens to the disk blocks formerly occupied by the files?

Getting back in service following this file loss is not a problem; I'm
just trying to understand how postgres gets into this state.

Jack Orenstein