Update.

Explorer ran good for a couple of days, and then the problem seems to have come back. But not as bad.
I ran a Key On, Engine Run scan, and got these codes.
#137........HEGO(HO2S) sensor fault, always rich.
#538
#536........Brake On/Off circuti not activated during KOER
#632........Overdrive transmission Cancel Switch/no action during self engine run test.

Does the 137 code mean the O2 sensor is bad? Since there are two sensors, how do I determine which one it is?

 

A further test today, involved a test drive with short series of drives. Vehicle stalled several times, sometimes with a bit of warning, sometimes with out. Warning would be a series of misses, rpm fluxuating.
Each time the engine started, though with a bit of cranking. When it started, it would run raggedly, unless the engine was raced. Often there would be an oder of fuel.
The way it is acting, I almost get the impression it is flooding out.
So, tomorrow we will test the fuel pressure regulator.