My 1993 Sierra 305 (133.4K miles) starts up fine and will run for a few seconds then it will stall like the key was turned off. I can keep it running by giving it gas but after it stalls it has no spark. I replaced the distributor cap, rotor and spark plugs last fall.

 

I assume this is throttle body injection. Try pouring a little gas in the throttle body and try to start - if you can keep it running doing that you have a fuel delivery problem. My 93 Suburban has done this three times and replacing the ICM in the distributor fixes it every time.
Do you here the fuel pump prime when you turn the key on?

 

I can hear the fuel pump prime, and I sprayed gas into the throttle body and no avail. I used a spark tester last night after it stalled again and it was getting no spark. The truck will start again after a little while. I had the truck running again today and I had it running around 1500 to 2000 rpm for a minute or two and it ran fine after. It idles at around 900-1000 rpm. I also noticed the rpm gauge was getting jumpy when I first started the truck.