Assigning values to custom fields in sales order line items

I've got this really odd problem. I've poured over my code and can't find the issue. The scenario is this, I need to open an existing sales order and assigning incrementing numbers to a custom field on the line item. So if I had a sales order with 3 lines, I would want 00001, 00002 and 00003 as the values in the custom field. The error seems to be on the line just above the 'Next' command. Whenever I assign the value to the custom field, it overwrites the value of the line items above it as well. In then