Time machine slow and stops
A few hunches based on non-TM file transfer errors I've seen on macOS:


I would use Disk Utility to scan the drive and repair errors. https://support.apple.com/kb/PH22243?locale=en_US (Usually I tell people to back up any critical data first, but I assume you have a semi recent backup :))
Is the problematic folder a symlink to an external drive by chance? (Rare but something I've seen.)
How is the external attached? Check/swap cables &amp; ports if you have spares available.