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Could be several things, or a combination.
Timing. Vacuum advance, or mechanical advance, or sloppy distributor.
Worn out timing chain.
Vacuum leak, intake to head, or carb.
Leaky carb at throttle shaft.
Bad spark plugs or wires.
Bad points or dist cap.
I'm sure there are ohter possibilities...
I'd start with a full tune-up and see what you find while you're doing it.
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1971 GMC 2500, 402/TH400 4.10 Daily Driver
Lafayette, CO
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