Sounds like your HD is taking a dump. First, try remounting the drive upside down and see if the trouble goes away for a while; if so, order a new drive before the bearings go completely on the old one. Run Scandisk or Error check on the drive to lock out any bad clusters. Listen for nasty noises...
The new self-monitoring firmware in modern hard drives gives reports like that before the drive dies. Replace the drive while you can still copy your files off it.