If you can, get access to a smoke machine to help you look for vacuum leaks in the SAI system.

Also, check all of those little check valves. They rot internally or chafe through underneath and cause vacuum leaks. Check the oil dipstick tube, breather hoses from crankcase to valve cover, to airbox.

And last, you could check MAF output.

 

something nobody has mentioned is the o2 censors. try and get a couple of used ones cheap and see if it fixes the problem. i had the same code and thought it was my o2 censors. found out it was the maf. a lean code either means your letting in too much air, its not not getting enough fuel or the car isnt measuring either/or correctly. the o2 censors ever been replaced? they like to go out at or after 100K...