Multithreaded DB connection for QGIS plugin

Question

I am using QGIS 2.18.21 and PostgreSQL 9.6. on a Windows 8 machine.

I am trying to do processing of a certain database task in the background, and I want the user to be able to continue to work in the meantime. The threaded class looks like:

class LoadCanvasStagingData(QThread):

def __init__(self, username, transaction_id, connection_string):
    QThread.__init__(self)
    self.username = username
    self.transaction_id = transaction_id
    self.selected_database = selected_database
    self.connection_string = connection_string

def run(self):
    # print ('\n\rsleeping')
    # time.sleep(0.05)
    # print ('done sleeping')

    con = psycopg2.connect(connection_string)
    cur = con.cursor()
    query = """ SELECT
                        *
                FROM qgis_validation_plugin.fetch_transaction(
                    '{0}'::CHARACTER VARYING,
                     ARRAY[{1}]::BIGINT[]
                 );
                    """.format(self.username, self.transaction_id)
    cur.execute(query)
    cur.close()
    con.commit()
    con.close()

It is called with:

 loadCanvasStagingData = LoadCanvasStagingData(self, self.username, self.transaction_id, self.connection_string)
 loadCanvasStagingData.finished.connect(self.refreshCanvas)
 loadCanvasStagingData.start()

It crashes on load about 75% of the times (with a .dmp file generation), seemingly unrelated to the data. But it does not crash at all if I uncomment the time.sleep.

Also, if I move the sleep thread to the bottom, it also does not seem to crash.

Does anyone know what might be going on?