To me it sounds like a head gasket leak.  On my '91 I put in about 1/4 can
of radiator stop-leak that said it seals head and head gasket leaks just to
determine if the thing was worth fixing.  That was about a year ago and I
have not had any problems since other than my heater core got clogged from
the stop-leak.  When the heater core was clogged the temp gauge was going
pretty high up there.  I had put in a whole can of stop-leak when the heater
core got clogged.

Supposedly you can buy test strips from auto parts store to check for CO on
the coolant which would indicate a head leak but I've never tried it.

I know it's a real PITA to get at a couple of those spark plugs but a
compression test that shows one, or two adjoining cylinders with low
compressiom would be another strong indication of a head leak.

When I had white smoke it was from a leaking intake manifold gasket.  This
is another common problem on the early Explorers.  If you do have to replace
the head gaskets then you'll end up replacing the manifold and valve cover
gaskets too and then everything should be perfect ;-)

My 92 did that years ago. The clue here was that when it started moving
gain, the temp dropped back to normal.
The answer turned out to be the fan clutch. The fan was slipping and did not
supply enough air when idling at a stop.  Once underway the air flow through
the grill was sufficient to return things to normal.

Charlie