I have a SUN L20 tape library runing as a media server
for veritas netbackup 5.1 on SUN solaris 9. the systems
runs however SOMETIMES performance is terrible. 

for example, a recent backup from a local disk to tape 
got 60KB/s in throughput.

enviroment : solaris 9, netbackup 5.1 MP3, drive cleaned regularly.

on media server /etc/system: 
set shmsys:shminfo_shmmax=4294967295
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmseg=10
set shmsys:shminfo_shmmni=512
set semsys:seminfo_semmns=2048
set semsys:seminfo_semmni=2048
set semsys:seminfo_semmsl=250
set msgsys:msginfo_msgmni=1024
set msgsys:msginfo_msgtql=1024
set semsys:seminfo_semopm=128

on media server  /usr/openv/netbackup/
   NET_BUFFER_SZ = 262144  in /usr/openv/netbackup
   SIZE_DATA_BUFFERS = 262144 in /usr/openv/netbackup/db/config
   NUMBER_DATA_BUFFERS = 32;

I even have an entry per veritis recommedation 
   SIZE_DATA_BUFFERS_DISK = 1048576
   NUMBER_DATA_BUFFERS_DISK= 16

can anybody offer some clues what is wrong here?

Thanks.
Jason