Xolox uses other users incomplete donwloads as an other source of files to speed up.
The problem is that sometimes xolox could be downloading a file part that is not part of the file you want! Then when it check the file it can't glue the parts together as they don't have the necessary overlaping bytes

You could try to enable (if not already so) "drop and retry incorrect downloaded parts" and "switch to SAFE download mode when mode then 10 downloade parts are dropped"
in Preferences/Downloads

You abort the download and try again from the begining. I know it hurts to abort a 700mb 90% download, but once I had a 300mb that would not complete for about a week... I started again and it has in my HD after only a overnight dl

Other that this i recomend gnucleus for big one-source-only downloads. But be pacient it will take time.

[]'s
Canalha