We had a similiar kind of problem, well not for Oracle but for Informix.
As pointed out earlier, majority of blame is to be attributed for the environment settings.
To check that,
make sure all the oracle bin and oradir are pointing to the right directory
export all the oracle env in a shell and run a specific script at a time from command line and redirect the errors (2) to a log, that would be easier to debug
if that is running fine, run the same job through cron
it would be less troublesome if you are running and testing everything on a test box which has the same configuration rather than on a prod node ( luckily
we had two such nodes )
most probably these should solve your problems!