System too lean means you're either not delivering adequate fuel pressure/volume or you are getting unmetered air in your engine. The options are to check your intake boot, DISA o-ring, and intake air temp sensor o-ring for damage and to replace your fuel filter and pump. If these don't solve your problems, then look into more expensive solutions like a faulty MAF sensor or faulty fuel injectors (unlikely because it's the entire engine).
My money is on the fuel filter and pump. A broken intake boot doesn't come and go like this and will throw a code fairly quick.
Also, I'm guessing the code you read today will be the same as before.
Last edited by SamDoe1; 01-22-2013 at 10:06 AM.