Scans jamming

[Mika Arasola <nmap-list () arasola fi>]

I have a created a script that scans networks with nmap. I've scheduled it
so it starts a new network about every 12 hours or so (each network scanned
twice in the 12 hours), and it's run from cron. Occasionally some of these
runs seem to jam completely, and I can't quite figure out what is causing
it. It seems to me that it is not related to delays in the previous run (so
there are several at a time) but to make sure I have changed the script so
that each run has unique output files.

Looking now at the currently jammed scan it seems to have gone through 191
ip-addresses of class C network. No errors in the output and this is the
second run (meaning it has already completed the first more intensive scan).
I'm doing 12 scans per week and at least one of them jams (changes every
week).

Any idea what could be going wrong?

Thanks,

Nmap version 5.00 ( http://nmap.org )

host:
CentOS release 5.4 (Final)
Linux 2.6.18-164.el5 #1 SMP x86_64 x86_64 x86_64 GNU/Linux

Process:
root     10075  0.0  1.0  32768 21576 ?        S    02:46   0:00
/usr/bin/nmap -T4 -PN -sS -p
179,264,646,1581,1863,2123,3300-3400,3724,5222,5431,5432,6112,8002,8007,8008,8080,9090-9100,10022,10100-10110,10150-10160,30000-32000
--version-intensity 2 --max-retries 2 --open --reason -oX
/home/nmap/current/results/XXXBACK/lastscan-xxxxx.xml 76.xx.xx.0/24

Open files:
root     15043  0.0  0.0   8700  1004 ?        Ss   Dec02   0:00 sh
/home/nmap/current/scan_network.sh XXXBACK
COMMAND   PID USER   FD   TYPE DEVICE    SIZE       NODE NAME
nmap    10075 root  cwd    DIR  253,0    4096    2403521 /root
nmap    10075 root  rtd    DIR  253,0    4096          2 /
nmap    10075 root  txt    REG  253,0 3580248    1762491 /usr/bin/nmap
nmap    10075 root  mem    REG  253,0  139416     714584 /lib64/ld-2.5.so
nmap    10075 root  mem    REG  253,0 1717800     714763 /lib64/libc-2.5.so
nmap    10075 root  mem    REG  253,0   53880     714621 /lib64/
libnss_files-2.5.so
nmap    10075 root    0r  FIFO    0,6            1119093 pipe
nmap    10075 root    1w   REG  253,0   37163    1364948
/home/nmap/nmap-scanner-0.4/results/XXXBACK/temp/output.tmp
nmap    10075 root    2w  FIFO    0,6            1119094 pipe
nmap    10075 root    3w   REG  253,0   64026    1365110
/home/nmap/nmap-scanner-0.4/results/XXXBACK/lastscan-xxxxx.xml
nmap    10075 root    4r   REG    0,3       0 4026532237 /proc/net/route
nmap    10075 root    5u   raw                   1177591
00000000:00FF->00000000:0000 st=07
nmap    10075 root    6u  sock    0,5            1177592 can't identify
protocol
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