Laser either dirty or starting to go.Try cleaning laser with commerical disc cleaner,put a drop of rubbing alcohol on brushing.Laser can be cleaned manually with a Q tip dipped in rubbing alcohol but this requires opening burner.If you do this MAKE CERTAIN ELECTRICAL PLUG IS PULLED,lasers cause nasty burns.
Some times the problem is just the brand of disc,some brands don''t work in Liteon's.
If laser is going the burner can be replaced.Most of the Liteon 5001's used Liteon 401s burners,411s or 451s dvd burners all will work.The dvd burners mount from the bottom PC burners mount from the side so they would have to be adapted,many PC burners won't work in these older liteons some freeze up the second you hit fast forward.