Indexes can break and cause sporadic symptoms. Do you have a know good backup? I would dupe the backup, import the data and test.
https://www.google.com/search?q=repair+index+filemaker <- Repair Index
If you have FileMaker 11 or newer, you can use Advanced Recovery options to rebuild your file's indexes:
- With the file closed, select Recover from the File Menu.
- Select "Use advanced Options"
- Select only: "Copy File Blocks as-is" and "Rebuild Field Indexes Now".
- The recovered copy of the file will be identical to the original copy except that it has completely rebuilt indexes.