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Q:

My truck runs great until it gets into a hard pull.then it starts splutting . on 1999 Ford F-250 Super Duty

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It straightening out it doesn't do this all the time.
(2) Answers
Check fuel pressure under load if you can. If the fuel filter hasn't been replaced recently you could try that, it sounds like a fuel supply issue.
Is the check engine light flashing on and off when this is happening? If it is, then you have a serious misfire. If the check engine light is on, but not flashing, it could still be a misfire or something else like a fuel issue, but you should get the codes scanned to point you in the right direction.