The best answer would be that if you can have another volume to move the journal onto another volume. EXT4 sees much better throughput in that case.
What's probably happening is that you have a small database under some degree of general load, and autovacuum is running in the background. Autovacuum will analyze used tables and write data to the stats tables periodically.
If the load is low, you may be able to tweak your autovacuum settings to avoid the problem as well.