AD import failing less than half way through.

When importing from AD, Total Requesters in AD = 2667, the import fails less than half way through. The import worked 100% the first several times used but since then the percent of failed records is nearly always the same. I have sent the log(s) to the Support Techs but they can not figure out the problem. We have 6 domains I can import from and I have tried all 6 at different times (even the weekends and evenings) and nothing seems to help/work. We are using version 5.5.0. Importing from CSV is not an option as this only adds requesters and does not replace the exisiting ones.

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If anyone has any suggestions please let me know as it would be greatly appreciated.