Well I originally had the ps3 version of the game but ended up selling it because of the constant lag issue where the game became unplayable. A few months ago I decided to buy the pc version of the game and everything had been fine until the past few days. Now I am experiencing the same game-breaking lag that I had on the ps3 version. Is there any way to fix this on the pc version or did I waste my money for the second time? :\

Oh great Talos, please save my game before I grab an axe and go on a killing spree like a certain paranoid elf frokm Oblivion.

Delete your old saved games

Is it lagging in certain spots? When you do certain things? Are you running any mods? Saying "I have lag" is just too broad a statement for anyone to help.
Have you checked your inventory for the mysterious multiplying key issue? (which could have also been the problem on ps3)