I have a chevy 1500 wt version 4.6 v6 why does it bogg out sometimes

Some things to consider with very old vehicles; fuel pump/filter/regulator, catalytic converter, electrical/electronic systems, ignition system, throttle body and engine computer. Is the fuel pump original? What's operating pressure? Fuel filter replaced? Fuel pressure regulator operating or worn out. If this 1500 uses a throttle body injector with built in fuel pressure regulator, it may need to be rebuilt with a new injector and/or regulator diaphragm. Is the catalytic converter original, possibly disintegrating internally and blocking exhaust flow? A fast and simple way to test for a blocked catcon; remove the upstream O2 sensor for an alternate exhaust path and startup. Drive around. Is there an immediate power increase indicating the catcon strangling exhaust flow? Aging electronics may appear as intermittent spark, injector operation with normal engine running until heat builds up, affecting electronics. Wiring may be compromised from normal weathering, aging.