I've seen a string of the similar problem with '96 and '97 GM trucks lately with CSFI-style injection. Under the plastic upper intake plenum there is a central fuel injection "spider" that delivers fuel to the intake runners via plastic poppet valves. These tend to get clogged and stuck. The cheap route would be to give it a good soak cleaning to free up the poppets. Or you can replace the whole spider assembly with one that places the fuel injectors where the poppets were. Here is an example of the conversion:
http://www.theautoshop.net/VortecMPFI.htm
The fouled plugs mean you've got leaking seals, this will also cause misfiring and the cat problems (they may be shot due to all the unburned fuel going through them). You may also have carbon build-up in the cylinders and valve stems, causing predetonation. Grab some cans of Seafoam and treat the engine, oil, and fuel...use as directed! Then change the oil after 500 miles.
Then I'd check the plug wires for any cross-firing or arcing to ground, the distributor gear for excessive wear, and the intake for any vacuum leaks. A good cleaning of the MAF sensor can't hurt either. Hope this helps!