ignition problems?????

The 'ol red dragon has recently had some starting problems that are getting progressively worse. On start-up the engines seems to catch on only some cyliders, it has that sputtering car shaking diesel feeling. Some times if i hold the starter for 3 seconds or more i can get it running smooth. No matter how long i crank it if i stop then start cranking again it starts right up. The only time it seems to not have this problem *sometimes* is when parking and running in to grab something from a store for a minute or two. I have let it chill in ignition on fora few seconds prior to start ( the whole fuel pump priming thing) this seems to not help. i just replaced the plugs and wires ( old plugs looked like dog sh*t, huge gap and rusted/ brown threads. but i dont know what else could be the prob. thanx for the help

-kevin
98 4.0 sport aw4

Dump a bottle or two of Chevron Techron fuel system cleaner thru it, take it out on the hiway and run it some in the high end of the power band. Clean the throttle body but take it off and do it on the bench, keep the cleaners away from the sensors. Use some GM Top engine cleaner or Sea Foam on it.
See how that works.