Hi, thare would abe a few causes for something like this.. Things like vacuum leaks, clogged injectors, excessive carbon buildup on the valves, dirty MAF sensor, and low fuel pressure would all do this, as well as exhaust leaks and o2 sensor malfunctions.

I would start with cleaning the MAF, though.. There is an element in there and it heats up.. Well, the computer measures the amount of air that is entering the engine by how fast that element cools off.. If this element is dirty.. then it will not cool off fast enough, and the computer will think there is less air entering the engine than there actually is.. and it will cause the 02 sensors to pick this up as a lean condition. This is what it is 90% of the time, anyways.. If it is not that, then look it over real good for vacuum leaks, and check the fuel pressure.. an if you still can't find the problem, then it will need to go to a shop for the proper diagnosis with the scantool.

Hope it helps!