Is this occuring on the same file/s or randomly on any file. Secondly I am assuming that a script runs your backups. It should shut down, backup, then startup up the database/application. Are you sure that your database is actually getting backed up well. My concern is that this file/s may be calling for recovery because of inconsistency between them and controlfile SCN.
Try this,
Make sure you are not doing SHUTDOWN ABORT
before starting the backup. Should be at least "shutdown immediate"
Even if you do "shutdown abort" do "startup" again then "shutdown immediate" do the backup then "startup" again.
NB Your database should be in a consistent state (ie controlfile in sync with datafiles - a clean shutdown) before you do your cold backup.
Hope this helps.