[ubuntu] Software Update Failure messages

I am using ubuntu 12.04 LTS on a 64-bit Dell Inspiron 1525 (Intel® Pentium(R) Dual CPU T2370 @ 1.73GHz × 2). I have been notified that updates are available but the Update Manager is
failing. The error messages I get imply that my Internet connection is failing but I have tested it and it is working fine. The messages are:

Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]
Failed to fetch http://security.ubuntu.com/ubuntu/po...04.1_amd64.deb 404 Not Found [IP: 91.189.92.200 80]

I have been out of the loop for a while with health issues so I'm stumped. I have never seen so many system failure messages and unexpected program shutdowns as I have seen lately.
And these are basic programs: Software Center, LibreOffice, etc. Should I upgrade to a different version of the system? I thought LTS would be conservative and stable.

Any thoughts or opinions arewelcome. Thank you for your time.