problem with vsftpd with anon_max_rate option

[yk103]

Hi everyone there:
I get a problem with my vsftpd, It do works well when 
anon_max_rate=0.But after I set it to 500 or so, hoping to limit 
anonymous users max download rate,a weird problem occurs.
The download stops after several bytes downloaded(Tested in 100M LAN)

My vsftpd.conf  :
####################
anonymous_enable=no
local_enable=yes
write_enable=no
local_umask=022
dirmessage_enable=YES
xferlog_enable=YES
connect_from_port_20=YES
xferlog_std_format=YES
ascii_download_enable=YES
listen=YES
pam_service_name=vsftpd.mysql
tcp_wrappers=YES
listen_port=3005
guest_enable=yes
guest_username=ftp
anon_upload_enable=yes
write_enable=yes
anon_mkdir_write_enable=yes
anon_other_write_enable=no
max_clients=30
max_per_ip=3
anon_max_rate=500
#########################
BTW : I use pam_mysql moudle to enable virtual user.
Any help is appreciated!