Even if inserting the data multi-threaded, there are still internal operations like the scheduled updating free text indexes etc that can be occurring in the background, reading and writing to the tables hosting the RDF data. So you still should try changing the transaction isolation level or concurrency mode connecting with to minimise the locks and pending transactions as is done in the Transactional Bulk Loading of RDF Data into Virtuoso DBMS via the Jena RDF Framework post.
Are any errors being reported in the virtuoso.log file during the load operations ? It would also be worth enabling additional tracing with the Virtuoso trace_on() function to get more detailed information written to the log.
If the database is crashing sometime and a core file is being generated , when ulimit -c unlimited is set, if you are compiling Virtuoso yourself you can also rebuild a debug binary to enable core file generation for analysis to see where the crash is occurring as detailed in the Virtuoso open source “gdb” core file stack trace generation for analysis post.