My 2000 Windstar LX 3.8 L has just passed the 56,000 mark. The check engine light came on about 3 weeks ago. I went to AutoZone for a scan and it came back P(0171) System too lean (Bank 1). There was no noticeable problem at the time so I reset the computer. About two weeks ago the engine started to act like it wanted to stall (hiccup) when in drive and at a stop. It has not stalled yet and the hiccuping comes randomly. There is also no check engine light. I took it in for service and the garage replaced the mass air flow sensor. It ran well for a few hours but it is now as bad as it was before I took it in for service. I have also replaced the idle air control and PCV within the last 8 months. Anyone have an idea what could be causing this?

I'm guessing "unmeasured air", (or aka "vacuum leak"). It will probably get bad enough eventually to give you a rather solid CEL. As you probably already know, the intake manifold seals may have to be replaced ... not a big job.