Long post with little useful information:

1. What oil does it have in it?
2. When was the last oil change?
3. What kind/brand oil filter?
4. Have you put on a known good/accurate oil pressure guage to confirm
the reading?

I'd guess it was running cold, never warming up. Then the new
thermostat let it warm up and now the oil is thin, and it can't keep
oil pressure.

Maybe a heaverweight oil might give you a few more miles.

Same thing happened to my '96 two years ago.  Replaced the oil pressure
switch and resolved the issue.  The part was $40 if I remember correctly.

Ralph

It sounds to me like they forgot to connect something when they re-installed
the intake.

Was the temp guage jumping around as well.  That is usually more of an
indication of low fluid rather than a defective thremostat.

This is an indication of a worn engine.  What weight oil are you using?  If
you are using 10/30 or lighter, you can probably buy more time upping it to
10/40.

The next time that it does this, put it in neutral and slowly step on the
gas while watching the guage.  If the needle slowly comes up as you increase
RPM's, then as said before, the engine is just wearing out.  If the gauge
suddenly jumps back to normal. it could indicate a bad oil pressure sensor
and I would have that replaced first.  It is not overly expensive and will
confirm what the real problem is.

The blue smoke is being caused by either the rings or valve seats and
guides, depending on when the smoke occures.  Start-up smoke is usually
caused by worn valve seals and guides and smoke while running is usually
caused by worn rings and/or cylinder walls.  The plenum gasket repair has
nothing to do with either.

The symptoms that you described and the situation where it occurs really
sounds like a tired engine.  The oil pressure is dropping at idle when the
engine is hot but it may not be dropping as much as the gauge indicates.
You can replace the sensor and see if anything changes and you can increase
the viscosity of the oil to buy yourself a little more time.

You could always have the engine rebuilt if the rest of the truck is worth
the expense.  The way I do it is to figure out how much the project will
cost and then take a good look at the vehicle and determine if I could get a
replacement in the same condition for the same or less money.  If so, then I
don't bother as I would be wasting money.  Good luck.

Why anyone would do an intake gasket job and -not- replace the
thermostat is beyond me...

Sounds like this may be a high mileage engine.

Any of the three is quite a stretch.

If they used Roloc discs to clean the gasket surfaces, the debris
from the Roloc discs will more often then not find its way to the
bearings and destroy them.  Oil analysis and autopsy of the oil
filter will reveal whether this has happened.

Or, it could simply be that the oil pressure sending unit has
worn out and needs replacing.

Does your repair order from the intake gasket job show that the
engine oil and filter was changed ?  It should/better have been!

How often has the oil been changed? regularly? as in every 3,000 miles?
put a manual gauge on it and run it for long enough to get it acting up,
if the manual gauge shows a problem then drop the pan and look. i suspect
you will find sludge particles floating around in the oil pan, these get
picked up in the oil pump's pickup screen little by little. then you find
a dropping or erratic oil pressure. most common is the dropping oil
pressure. If you find sludge your best course of action is to replace the
engine assy. the next best would be to clean the pan, replace the oil
pump and pickup tube then hope you can last a while before enough of the
sludge up top gets dislodged to start causing you problems again.

When i was working on cars i replaced more than a few engines for this
very thing.

another guess would be that you have a leaking oil pressure sending unit.
I've seen them leak into the plastic housing and after its warmed up good
give all kids of erratic/wrong readings before again this can be proven
with a visual inspection and mechanical oil pressure gauge.

Thanks to all who gave some insight.

I just had the oil pressure sending unit/switch? *behind the engine*
replaced.

Yes, oil change has been done every 3,000 miles as I have had problems
before
with the oil pickup screen getting clogged. That was replaced last year
along with
the oil pump.

Last place that worked on it changed the oil and put 10W40 in it.  I am
wondering
if the place before them actually did any work because the last place I took
it to
said the thermostat was not replaced and had not been changed in quite some
time.
That was based on the way the gasket was when it was taken off.

As for the blue smoke, I am thinking about having the piston rings redone on
my
extended warranty and since they have to take off the top part of the engine
to
replace the valve seals and guides while they are in there and accessable.
Any
other gasket they can get to, might as well replace those.

Just today on errands, after it was warmed up, the needle would slowly drop
to
just above the lower mark and stay there.  But then sometimes at idle, it
would
be just enough for the light to flicker on / off.  I did slowly give it gas
at idle but
the needle did not jump.  Got it up to 2000 RPMs and it slowly got up to
just
below 40 and then after letting off the gas, it slowly dropped again.

Needless to say, I think the piston rings, valve seals and guides and
another
plenum gasket will need to be done and hopefully they will put it all back
together
correctly.  As I have a feeling the last place did not do it right.  As for
the oil
pressure sensor, since a new is on, maybe it is a bad one or something
internaly
in the engine. The last place told me they put a gauge on the sensor and it
matched what the needle was showing.

Thanks for the great comments.... anymore to what I have included above?