Engine misses...

[maxharley]

I just purchased a 2004 F-550 with a 6.0, it has 52,000 mile on it. It starts fine and runs good cold, but when it warms up and under load it sounds like it has a miss. It still has decent power but does not sound or feel smooth. I tried a bottle of sea foam in the fuel but it did not help. Any ideas what it might be ?