Ford Repair: eratic idle and voltage surges, voltage surges, oxegen sensor

Question
My 1989 Ford F150 PU has a 5.0L. It is EFI EEC IV with a AOD automatic transmission.   Cold starting it cranks right up and idles up a little high and soon idles down. Runs fine. Then I drive it after it warms up. I come to a stop and at that time it will have an eratic idle.Up and down.At the same time it sometimes shows to be having voltage surges on my volt meter. The service engine light works but does not come on.I changed the oxegen sensor. Still the same thing.I hope you can tell me what is wrong with it. It is getting poor fuel mileage also. It has a new major tuneup inclueding fuel filter. Im puzzeled.It will do it most of the time but not all the time.IT has plenty of power at all times.I changed air filters cleand throttle body.Hope you can help me out on it.Its one owner and well maintained and uses no oil.  Thank you for your time and insight.  

Answer
Most likely, your idle is surging because of a vacuum leak, a clogged breather, or stuck PCV valve. The volt gauge is fluctuating because the idle is surging.  The Service Engine light will only come on if an emission sensor is not operating in its normal voltage range.  It is possible that a sensor is bad, but is still within its operating range.  The filters and spark plugs were probably needed, but wouldn't cause a surging idle.

I hope this helps,
Dave