A bit of a wild guess from me: could it be that “Print Message When Reference Is Not Found” is enabled?
300 records should be easily doable for the importer, so something must be going on in your case. Enable statistics and see if you can pinpoint a record where it happens. (try 150 records, if that works try 200 etc)
You may want to start eliminating steps in the template: remove all columns but one for example, and see if that works. Then extend to see where it breaks.
I hope this helps.