if it gradually got worse, there is probably a leak somewhere from the throttle body to the head. the extra air sucked in will flow in increasing your idle until the engine tried to compensate back to normal then the air comes in again and keeps repeating the cycle. check your aic valve (make sure there is a rubber o-ring on it, check the throttle body to manifold gasket/bolts, check the manifold to head. since the extra air is sucking into the engine it will have too much air causing your "exhaust system too lean" code. hope this helps