1. Put your txt file into a csv file.
2. Add a Read file, and then read your txt file to a Table variable table 1.
3. Add a Loop command to read the front 1000 lines out, and then save them to a Table variable table 2, also delete them from table 1.
4. Write the value from table 1 to overwrite the original content in the csv file.
You can edit some steps according to your needs.