Idle is going up and down on its own

2001 mercury cougar 6cyl idles lows in park or at stop sign sometimes will die. Check engine light comes on running lean on bank 2 no apparent hoses with cracks or holes what could it be? When my car is turned on it idles up an down it will sometimes cause it to die. When the mechanic put it on the tester all that came back was running lean on bank 2. He said he couldn't find any bad hoses and said he cleaned some sensors then sent me on my way. Well I drive it like 3 miles and the check engine light came back on and it died when I pulled in to park. It seems to run fine when I am driving from what I can feel. I listened for an air leak couldn't hear anything.