"Run system command cron instead of PHP cron. Please note that this option may be enable only if listings are not being published but added to Draft status."
Check you _memorylimit and _max_executiontime on your server. Probably you need to increase one of them or both. If this can't be done, you need to divide your schedule in to the parts using Property Query Field(for Rets feed) :
Contact your host provider and ask why are you running into issues, they will need to investigate further. Check your error log file for errors related to Importer.