My gmc 2500 diesel pickup engine keeps cutting out 1989?

when you say fuel pump, was it the injector pump, or the lift pump??? If it was the injector pump, the injector lines would need to be bled, but since the truck is already running.... If it was the lift pump, there would be air in the system, and the truck would blow white smoke for a short time till the air worked its way out of the system. You could have the the injectors pop-tested, and check the tips. How many miles are on the truck? Maybe the injector timing needs to be adjusted slightly, as it may not be as it was new (worn parts). Is this a recent problem? Maybe you got some bad fuel/excessive water. Try some STA-BIL for diesel engines to absorb water.