Warmer weather will reduce the severity of such a problem -- exactly!

This did not seem right to me so I took a quick look at all possible
engine codes for a '92 model 5.0 liter Ford PU.  I didn't find a "low
fuel pressure code."  There is nothing on that engine that directly
monitors fuel pressure.  There is a code for "oxygen sensor indicates
system lean."  Low  fuel pressure IS ONE POSSIBLE CAUSE OF MANY for
this code.  This code can easily be caused by a vacuum leak.  Vacuum
leaks cause exactly the bad and even possibly "hunting" idle that you
describe.   At idle the rogue air coming through a vacuum leak is a
high percentage of the total air intake and greatly disturbs the
mixture.  At wide open throttle it is a drop in the ocean and so --
despite common false belief -- the vaccum leak has little or no
effect.

Correct -- probably not a fuel pressure problem if the truck winds out
with vigor.

Good idea!

This truck probably has a vacuum leak at the intake manifold that
closes with warm up.  VERY typical.  We have found this situation hard
to diagnose until we got our smoke machine which makes it child's play
to find intake manifold leaks on a cold engine.  We hook it up to an
engine that has sat overnight and run the test without ever starting
the vehicle.

Less likely is a spark plug that was dropped during installation or
not checked and has an almost closed gap.  It will light the fire at
high rpm but not at idle.  The dilute fuel mix at idle requires a wide
gap whereas wide open throttle much less so.  One reason this is less
likely is that if this is the cause the problem would be expected to
persist when the engine warms up.

Don
www.donsautomotive.com

First, as Don indicated, there is no fuel pressure sensor on
that vehicle - therefore, no code possible.  That vintage
did have problems with fuel pressure regulator failure which
can and does cause idle and low fuel mileage problems.  If
you have fuel in the vacuum hose to it, don't bother to
check the pressure - just replace it.  They are not
adjustable unless you buy a specialty performance unit.
Your most likely idle problem is the IAC and a filthy
throttle body.  I know it has a tag on the side saying not
to clean it but, there comes a time that it has to be
cleaned.  If you remove the coating, it only means it may
have to be cleaned again sooner than normal.  Use a good
throttle body cleaner.  Ford has a good one as well as some
in the aftermarket.  It is also possible to remove the IAC
from the side of the throttle body and clean it making it
return to satisfactory operation.  It may require removing
the motor part and soaking the valve portion in a good
solvent overnite.  If that doesn't do it, you will need to
replace it.  The IAC will not usually set a fault code if it
is only dirty - just cause low/erratic idle.  BTW, just
spraying carb cleaner into it will not do the job.  You need
to just remove it.  The throttle body can be cleaned pretty
well with a couple of old tooth brushes while holding the
throttle wide open - do this with the engine off!

Good luck
Lugnut