I would like an opinion on what might have gone wrong?

Yesterday midnight my server 'hang' for the second time.
The first time (abt a month ago) was similar scenario, my log stops around midnight.
The next morning, I don't even see anything on my screen but to reboot the server.

I would like to know in this kind of circumstances, is it the hardware or software problem? I also noticed I have sound and USB modules loaded which I don't need. I will probably remove them later.

I have no process that was schedule to run after midnight.

Jul 18 00:48:41 myserver -- MARK --
Jul 18 09:04:40 myserver syslogd 1.4.1: restart.

I am using Slackware 9.0, kernel 2.4.21

For those who want to see my hardware specs, I have included my pci devices, cpuinfo, meminfo, modules loaded
***************************************
PCI devices found:
Bus 0, device 0, function 0:
Host bridge: Intel Corp. 82845 845 (Brookdale) Chipset Host Bridge (rev 4).
Prefetchable 32 bit memory at 0xd0000000 [0xd3ffffff].
Bus 0, device 1, function 0:
PCI bridge: Intel Corp. 82845 845 (Brookdale) Chipset AGP Bridge (rev 4).
Master Capable. Latency=64. Min Gnt=6.
Bus 0, device 30, function 0:
PCI bridge: Intel Corp. 82801BA/CA/DB PCI Bridge (rev 5).
Master Capable. No bursts. Min Gnt=14.
Bus 0, device 31, function 0:
ISA bridge: Intel Corp. 82801BA ISA Bridge (LPC) (rev 5).
Bus 0, device 31, function 1:
IDE interface: Intel Corp. 82801BA IDE U100 (rev 5).
I/O at 0xf000 [0xf00f].
Bus 0, device 31, function 2:
USB Controller: Intel Corp. 82801BA/BAM USB (Hub #1) (rev 5).
IRQ 11.
I/O at 0xd000 [0xd01f].
Bus 0, device 31, function 3:
SMBus: Intel Corp. 82801BA/BAM SMBus (rev 5).
IRQ 5.
I/O at 0x5000 [0x500f].
Bus 0, device 31, function 4:
USB Controller: Intel Corp. 82801BA/BAM USB (Hub #2) (rev 5).
IRQ 9.
I/O at 0xd800 [0xd81f].
Bus 0, device 31, function 5:
Multimedia audio controller: Intel Corp. 82801BA/BAM AC'97 Audio (rev 5).
IRQ 5.
I/O at 0xdc00 [0xdcff].
I/O at 0xe000 [0xe03f].
Bus 2, device 0, function 0:
Ethernet controller: 3Com Corporation 3c905C-TX/TX-M [Tornado] (rev 120).
IRQ 11.
Master Capable. Latency=32. Min Gnt=10.Max Lat=10.
I/O at 0xc000 [0xc07f].
Non-prefetchable 32 bit memory at 0xd5800000 [0xd580007f].
Bus 2, device 2, function 0:
VGA compatible controller: S3 Inc. 86c764/765 [Trio32/64/64V+] (rev 0).
IRQ 12.
Non-prefetchable 32 bit memory at 0xd5000000 [0xd57fffff].

processor : 0
vendor_id : GenuineIntel
cpu family : 15
model : 2
model name : Intel(R) Pentium(R) 4 CPU 1.80GHz
stepping : 4
cpu MHz : 1814.053
cache size : 512 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
bogomips : 3617.58

total: used: free: shared: buffers: cached:
Mem: 926294016 920518656 5775360 0 30326784 808304640
Swap: 73990144 20480 73969664
MemTotal: 904584 kB
MemFree: 5640 kB
MemShared: 0 kB
Buffers: 29616 kB
Cached: 789348 kB
SwapCached: 12 kB
Active: 49736 kB
Inactive: 818684 kB
HighTotal: 0 kB
HighFree: 0 kB
LowTotal: 904584 kB
LowFree: 5640 kB
SwapTotal: 72256 kB
SwapFree: 72236 kB

Module Size Used by Not tainted
ipt_REJECT 2968 1 (autoclean)
ipt_LOG 3384 1 (autoclean)
ipt_state 536 2 (autoclean)
ip_conntrack 18120 1 (autoclean) [ipt_state]
ipt_limit 856 1 (autoclean)
iptable_filter 1644 1 (autoclean)
ip_tables 11768 5 [ipt_REJECT ipt_LOG ipt_state ipt_limit iptable_filter]
uhci 24528 0 (unused)
usbcore 58432 1 [uhci]
i810_audio 23848 0 (unused)
ac97_codec 10696 0 [i810_audio]
soundcore 3332 2 [i810_audio]
pcmcia_core 40064 0
ide-scsi 9424 0
3c59x 26640 1

Do you have a cron job that starts at midnight?

Or perhaps, do you have a cron job that starts at midnight about once a month?

Yes, I have two monthly cron jobs scehdule to run 1st day of the month start at 12:30am. One for mail statistics, another backup using rsync.

However, when I ran it individually, it has no problem. I have another similar server which have these two cron jobs running without problem too.

I did not remember the first time it happens, but I will see if it will happens again in Aug 17 to be exact.

The problem could be a conflict between the two cron jobs, i.e. each works OK but when they run at the same time something bad happens. You could try running them one after the other instead of simultaneously.