[Bug] Corrupted download data

[IOftpd_USER]

I have strange problem - I have ftp box with Win XP SP3, no firewall, no antivirus and sometimes (randomly), when I download data from that box, data are corrupted. I found out, that file is bigger then it should be.

For example:
Correct file has 20*000*000 bytes, downloaded file 20*001*460 bytes. When I compare these two files, I can see there is three empty lines:


Logs:
Debug:

08-04-2009 08:45:45 Created TCL Interpreter #1
08-04-2009 08:45:45 Created TCL Interpreter #2
08-04-2009 08:45:45 Created TCL Interpreter #3
08-04-2009 08:45:56 Created TCL Interpreter #4
08-04-2009 08:46:04 Created TCL Interpreter #5
08-04-2009 08:49:10 Marked for deletion TCL Interpreter #1
08-04-2009 08:49:10 Deleting TCL Interpreter #1
08-04-2009 08:49:10 Finalizing TCL Interpreter #1 for thread 956
08-04-2009 08:49:10 Created TCL Interpreter #6
08-04-2009 08:49:11 Marked for deletion TCL Interpreter #5
08-04-2009 08:49:11 Deleting TCL Interpreter #5
08-04-2009 08:49:11 Finalizing TCL Interpreter #5 for thread 1688
08-04-2009 08:49:11 Created TCL Interpreter #7
08-04-2009 08:49:11 Marked for deletion TCL Interpreter #3
08-04-2009 08:49:11 Deleting TCL Interpreter #3
08-04-2009 08:49:11 Finalizing TCL Interpreter #3 for thread 1676
08-04-2009 08:49:11 Created TCL Interpreter #8
08-04-2009 08:49:17 Marked for deletion TCL Interpreter #4
08-04-2009 08:49:17 Deleting TCL Interpreter #4
08-04-2009 08:49:17 Finalizing TCL Interpreter #4 for thread 1580
08-04-2009 08:49:17 Created TCL Interpreter #9
08-04-2009 08:49:30 Marked for deletion TCL Interpreter #2
08-04-2009 08:49:30 Deleting TCL Interpreter #2
08-04-2009 08:49:30 Finalizing TCL Interpreter #2 for thread 1680
08-04-2009 08:49:30 Created TCL Interpreter #10
08-04-2009 09:00:16 Created TCL Interpreter #1
08-04-2009 09:00:16 Created TCL Interpreter #2
08-04-2009 09:00:18 Created TCL Interpreter #3
08-04-2009 09:00:18 Created TCL Interpreter #4
08-04-2009 09:00:26 Created TCL Interpreter #5

Error:

07-29-2009 16:32:22 AcceptEx() failed with error: 64
07-29-2009 16:32:22 AcceptEx() failed with error: 64
07-29-2009 16:32:22 AcceptEx() failed with error: 64
07-29-2009 16:32:22 AcceptEx() failed with error: 121
07-29-2009 16:32:22 AcceptEx() failed with error: 121
07-29-2009 16:32:22 AcceptEx() failed with error: 64
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:32:57 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:29 AcceptEx() failed with error: 121
07-29-2009 16:33:42 AcceptEx() failed with error: 121
07-29-2009 16:33:42 AcceptEx() failed with error: 121
07-29-2009 16:33:42 AcceptEx() failed with error: 64
07-29-2009 16:33:42 AcceptEx() failed with error: 64
07-29-2009 16:33:42 AcceptEx() failed with error: 64
07-29-2009 16:33:42 AcceptEx() failed with error: 121
07-29-2009 16:33:42 AcceptEx() failed with error: 64

xferlog is showing this file with no difference then the others.

When I re-download file manualy, it is usually ok. What can cause this?