DVD Navigation error

Hello, I purchased a LG LRH780 DVD player/recorder November 2006, (Canadian model) and it continues to have trouble playing DVD's, (store purchased, not recorded). It is a sparatic problem where it does various things like:
-stop playing intermittently throughout the playback
-skips or pauses
-wont load the disk and says navigation error
Sometimes when it pauses, you can't do anything except turn the power off and start over.
Please let me know what I can do.

Thanks