I agree on the fuel pressure:

But, on the transmission slipping from second to third. If it's a 4L60E or
4L65E, there is an issue with the pressure control valve for the lock up
torque converter.
I would have this checked, before it costs you a total rebuild. With lots of
hard parts.

Now you can just have the valve body machined, or use a shift kit, with an
insert type valve.

RK

cover wrote:  (1997 GMC 350 Engine Performance)

My GMC is cutting out under load.   It occasionally
misses in 3rd gear and OD but not in 2nd.   It does
this at around 2300 RPM, generally on a slight uphill
grade.    When it misses it feels like a sharp wind
hit the vehicle with a momentary dip in the tach
reading). It idles smoothly (dealer tuneup 5k miles
ago).      Any ideas?
________________________________________

Wires.        Fuel Filter.

Don't jump to "emissions type devices" or anything
   else until you cover the basics.               dahpater
________________________________________

I had a similar situation.  It turned out that I was
losing fuel pressure inside the fuel tank.    At less
than full load, all was well.   Max load caused a
stumble.   Isolation hose between pump & delivery
tube leaked by hose clamp.                       "syoung"
_________________________________________

It's probably not the fuel delivery system because it
only happens at part throttle.

It's probably not the high voltage system (wires, coil,
distrib, plugs), as it doesn't happen at full throttle.

Some other possible causes:

-- Sticking IAC
-- Intermittent Igniton Module failure.
-- Trans downshifting late.
-- TCC engaging too soon under load.

Trans and TCC errors could result from erroneous load
sensors such as manifold vacuum or throttle position.

Good luck.

Rodan.