By "sometimes" I mean it happens with certain files every time, but with other files not at all.
I get the same problem using Copy from menu, not just drag-and-drop.
Session protocol : SSH-2
FTP protocol : SFTP-3
Winscp version : 4.0.5
I'm not able to reproduce the problem.
What does it mean sometimes? Is the problem intermittent? Or does it happen with some file only? I mean does it happen whenever you download file DOWNUTIL2_SUN_6137663028.txt?
Also does it happen if you download files by other means that drag&drop? E.g. from menu?
What protocol are you using?
What version of WinSCP?