I think I found a work around. There appears to be a bug in the arcgis python module in the gis\__init__.py file in the _add_by_part method.
it seems that in line 3128, the script attempts to delete the split.part file before the thread completes.
I added a try block and it appears to run properly and appears to remove all split.part files in the temp directory. I acknowledge this isn't a clean solution, but allows the script to execute.