Lately my Ubuntu 12 system has started acting very strange. Sometimes the computer freezes and then unfreezes 2 minutes later, and other times when I try a basic command like less I get the error bash: /usr/bin/less: Input/output error. But this is intermittent too. Any suggestions?

Also if I try sudo reboot and enter my password, I get

sudo: unable to open /var/lib/sudo/plato/7: Read-only file system

Before I used to be able to do sudo reboot fine.

If I tail /var/log/syslog I do see these curious lines:

.... ata1: softreset failed (device not ready)
.... ata1: hard resettting link
.... ata1: link is slow to respond, please be patient (ready=0)

What can I do to fix this?