My server crash please help

My server is crashing and I cant figure out why, Its running Ubuntu server 6.10 And ISP Config 2.2.15 I installed following the perfect ubuntu setup inst.
I take a look at the logs but the only thing that is strange to me is this:
Sep 12 19:19:26 globamarket authdaemond: PAM unable to dlopen(/lib/security/pam_foreground.so)
Sep 12 19:19:26 globamarket authdaemond: PAM [dlerror: /lib/security/pam_foreground.so: undefined symbol: pam_set_data]
Sep 12 19:19:26 globamarket authdaemond: PAM adding faulty module: /lib/security/pam_foreground.so
at syslog, userlog. I need to know where and what can i do to identify my problem in order to solve it
This is the lines from syslog when ubuntu reboot:
Sep 10 23:53:58 globamarket postfix/smtpd[17908]: disconnect from unknown[58.186.90.99]
Sep 11 14:22:41 globamarket syslogd 1.4.1#18ubuntu6: restart.
Every time is the same
Thanks

 

What are the outputs of

Code:

df -h

and

Code:

dmesg

?

 

The requested

The outputs

 

Seems to be a problem with your memory. How much RAM do you use?

Please use memtest ( http://www.memtest.org/ ) to check if your memory is ok.