Hi,
There is some algoritm in amule to check if a chunck is valid or corrupted. Sometimes, amule have to re-download part of file because of currupted data.
The amount you give : aprox. 18 Mo is the size of 2 chuncks (each one usually is a bit more than 9 Mo). So you better check in the logs window if there is messages about corrupted chunck.
Or this maybe a real bug...
Hope this will help.
Regards.