K-21 engine?
There is a "last chance" filter screen located in the injector holder body. It's under a 2-screw cover plate on top of the injector holder body. This screen will over time clog from heavy ends/tar in the fuel vapor and starve the fuel flow.
When maximum acceleration is called for, that is when fuel demand is the highest, and also when you will notice the spluttering you refer to.
If the screen is coated when you take off the cover, I suggest you remove the injector holder from the truck and turn it upside down to remove the screen for cleaning or replacement.
Trying to remove the screen while the injector holder is mounted to the engine can result in some of the trash and crud in the filter bore managing to just fall right on into the bore past where the filter is located. This trash will then be carried right into the injectors.
The screen is held in place by a small circlip.