I am posting this here for mechanical novices or people like myself with
just enough knowledge to be dangerous.  Here is a hard learned lesson in
not checking/fixing the obvious first.  For the most part, no trouble
codes were available to diagnose the problem.

Around June 2005, I noticed my '93 Jimmy was running a little rougher.  I
figured, okay, time for new plugs, or wires, or something.  I read a bit
on the internet and heard about the Central Port Injector "spider"
problems, but I really didn't want to believe it was anything so
expensive.

So, I changed my plugs, and wires, and distributor cap and rotor.  They
were a little old.  After that it seemed that it ran smoother.  It could
have been an example of the placebo effect.  Things continued on this way
for months, it was a little rough at idle, but it seemed to work fine
once you got going.  Somedays, there would be no trouble at all.

Around November, I got my first and only trouble code, EGR valve.  I took
it to a local garage, and was informed that the EGR valve was all
carboned up and seized.  First expensive repair, about $500 (part+labor).  
Things seemed to run better after that, and this time, it was probably
for real.

Finally about 3 weeks ago, it got noticably rougher at idle.  Things were
fine when cold, but when warmed up, it would have a lot of trouble.  Hot
starting was quite difficult.  It threatened to stall when stopping.  
This was all probably due to fuel leaking into the injector manifold bowl
and getting evaporated into the engine.  A "carburator" in a fuel
injected engine. I of course balked at the possibility that it was the
CPI unit.  I also noticed about 2 months ago that the oil pressure would
go down a little more when stopped.  Not below "safe" levels, but
still... This was probably from uncombusted gasoline getting into the
crankcase via blowby, and thinning the oil.  So, I changed the oil again
(about a month ahead of my normal 3 months schedule), and changed all the
plugs, figuring that perhaps the "champion" plugs were lower quality.  I
put "genuine" AC Delco plugs in.  Of course, the problem did not go away.  
I also thought that perhaps an accessory was causing the engine to slow
down at idle, so I took the belt off and checked everything.  The
alternator seemed to have some resistance, so I changed it.  Perhaps
unnecessary, perhaps just good preventative maintenance.

The rough idles/near stalling got worse, and I started reading up again.  
I completely forgot about the CPI spider issues until this last weekend.  
By this time, of course, no auto parts place could get parts in before
Monday, when I would have to go back to work all week.

On Monday, it was basically undrivable once warmed up.  So I finally had
to get it fixed, and by now I was finally convinced the CPI spider was
bad.  Weather and work conditions would not permit me to do it myself, so
I shopped around for a garage to do it.  The cheapest place I found that
didn't answer, "WTF is a CPI?" was actually a local GM Dealership.  The
total damage came out to about $1000 (Canadian) before tax.  Pretty
expensive, but better than the garage that wanted $1300 (including $45
for the air plenum gasket).  Things seem perfectly fine now.  The old
unit looked in real rough shape, the manifold was washed completely
"clean" on the right side where the regulator is, and a bit on the left
side too.  Of course all the residue and varnish that was in that side
had probably gone into the cylinders.  

Of course, there could be lingering problems.  Thinner oil resulting from
gasoline getting in there probably did some good wear to the moving
parts.  The right hand side was running rich, and the left lean, and I
doubt that this makes for a well balanced running engine.  I changed the
oil immediately after the repair, and will probably change it in a month
again before going back to normal intervals.  Perhaps the oxygen sensor
and the catalytic convertor took some extra wear they shouldn't have and
will cause problems down the road.  The EGR valve would probably have
been fine without the extra gas going through.

So the lesson here, is check the obvious thing first.  And if you have a
Vortec with CPI (VIN Code W, between 1992-1995) that takes a turn for the
worse, it's probably the CPI unit.  If there's no codes, well, it
probably isn't a sensor (went down this path for a while too).  Also, if
your oil level goes up, even slightly, you probably have  a CPI problem.  
You'll just pay out more if you try to look for other problems first.  If
I had replaced the CPI pod in the summer, I could have done it myself in
excellent weather, and probably saved about $400 in labor.