Hi Alan.
As often with the 3rd party plugins integration, it may be most effective to contact the maintainers directly via their plugin's page at the Moodle plugins directory. Not sure which of the available Turnitin plugins you have instaled though.
if they try to go back to the first part again the system becomes unresponsive
What do you mean by "go back to the first part again"? Do they try to hit the "Back" button in the browser?