I would think that 4- miles is a bit long before going into cloased loop but it is possible that the upstream o2 (s?) is junk. A cheap scan guage will "usually" tell yo if the upstream O2 is bad. At idle it should be switching lean/rich quickly. Sometimes they fail and get stuck one way or another and other times can get very slow.
Also check your for your belly pan leaking. As the oil warms up/oil thins and more vacuum leaking...
You did check fuel pressure driving down the road at WOT right?
