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

It straightening out it doesn't do this all the time.

Asked by for the 1999 Ford F-250 Super Duty
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.
1 more answer
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.