Not sure what's causing it.

[sand41]

05 F250 FX4
The truck runs good but at idol the motor feels like it bumps every so often that's the best way I can describe it, today sitting in the driveway I brought the rpms up to 1500 and held it there until I felt the bump when it bumps the rpms dropped to about 1000 and then slowly climbs back up to 1500 rpms, it did this 3 times in just a couple minutes, is this an injector going bad or something else?

Thanks for your help.