Don't know why this works - but it does ...
Symptom:
Downloading file ... gets to 99.9%, then cycles - repeatedly - downloading final 2-to-5% of file. File eventually downloads after 10 mins to an hour (YMMV)
Tried:
Pausing download, verifying file, restarting Transmission: Behaviour continues
Solution
Pause the transfer, find in Finder, duplicate the file in-situ, rename original (just in case), rename duplicate to that of original, un-pause transfer.
File downloads quickly after one pass.
I hope this helps (and am wondering why it does).
Regards,
iMikeyMike