GMC Repair: 1994 GMC 3500 6.5 DIESEL 4+4, gmc 3500, computer link

Question
NOT ALL THE TIME BUT ON A 30 MINUTE TRIP AFTER THE TRUCK HAS SET ABOUT 45 MINS. TO AN HOUR IT CRANKS FINE RUNS 4 OR 5 SECONDS AND DIES THEN IT TAKES 4 OR 5 SECONDS TO REFIRE.IT ALSO SOMETIMES MISSES WHEN YOU REALLY GET ON IT OR IN A PULL.I HAVE DRAINED THE WATER (NONE) REPLACED FUEL FILTER AND FUEL PUMP.ANY SUGGESTIONS?

Answer
Hi
Does the service engine soon light ever come on?
If possible, could you take it somewhere and get it scanned for codes? On that unit, you could "flash" the codes, by jumping 2 pins on computer link.
Please try to get codes, it'll REALLY help you first, and me.
While it's missing, what color is the smoke? White? Black?
I would really hate to make a guess with out just a bit more info.
Thanks
Pawl