I'm assuming that you have a TBI system - correct?
With TBI/fuel injection there is no fuel supply buffer (like the bowl of a carburetor). So if you have a fuel flow problem caused by a restriction - like a fouled filter - it doesn't take a while to effect performance.
If the pressure/flow at the injector rail/TBI inlet is below spec, you will notice a lack of throttle response immediately out of the driveway. Also a fuel supply problem due to a fouled filter does not go away - and then return 1/2 mile after a "rest period".
I am sure you have done all the fundamentals (e.g. checked for loose harness connections/damaged wiring insulation, located repaired any vacuum leaks below the throttle body, made sure the inlet air filter is clean, checked the exhaust system for any obstructions, etc.) - so we can skip them.
You note that throttle response is diminished after about 1/2 mile of driving.
1. Is the speed of the vehicle a factor or just distance? IOW - say you drove only 10 mph (as opposed to 50 mph) - would you have to go a longer distance before the truck starts to bog down?
2. Or does the speed of the vehicle appear to have no effect on when the trouble starts? It just hits after a half mile regardless of how long it takes to go that far.
3. Another possible factor is running time; what if you just sit in the driveway and let the truck idle for an amount of time that equals the time it normally takes you to travel 1/2" mile - would the truck have acceleration issues as soon as you hit the street?
BTW - on this and any other request for help in diagnosing a problem - it would help if you could provide some detail regarding your personal history with the truck. Also; a brief timeline as to how this issue developed is helpful (i.e. just started one day & you've had it since, started slowly and kept getting worse, you got the truck and it was already acting this way, etc.)
