Oracle (32 bit) on Sun SPARC Solaris 2.8

We have a system where one of the main requirements is to
load incoming files into the database as soon as possible
after they arrive. The load is done into a partitioned (range partitioning)
table by 9 scripts running in parallel from cron. The table has no indexes
and loads are done using SQL*Loader with DIRECT=TRUE and PARALLEL=TRUE.
It works fine most of the time and I can see from V$SESSION that loads
really run in parallel (up to 5 of them - I have never actually seen all
9 running in parallel). But from time to time it comes with ORA-00054
"resource busy and acquire with NOWAIT specified". Loads run in
"table" mode (i.e. we don't specify a specific partition in
control file, although most loads actually go into one partition).

Is there a limit on the number of parallel loads that
can go into a single table?