I have a 2013 5.0 STX. 67k miles. Aftermarket exhaust was installed by previous owner. I installed larger tires (275/70-R18) and an SCT Tuner from AmericanMuscle to tune/compensate for the MPH.
Truck ran fine for a long while with tune installed, so I dont beleive the tune to be the issue. The issue i'm having is the engine will rev up and down about 200 rpms whethere at idle, partial throttle, or when in cruise control. Any ideas on what it could be?

 

As a test, remove the tune, put it back to stock.

 

Start with the easy stuff,

disconnect the battery and turn the headlights on to reset the factory computer to its baseline setting. I wouldn't fool with removing the tune yet.

Clean your MAF

Look for a loose PCV or other vacuum hose on the intake.

Turn off the headlights and reconnect the battery.

A wandering idle typically comes from the PCM getting conflicting info from the MAF vs the primary O2 sensors. Basically, the computer is chasing its tail because a leak or bad input value is throwing off the mixture.

 

Perfect, I'll definitely try those things Wookie. I tend to agree that it's not the tune, since it's doing doing it 100% of the time. I'll report back with what I find.

 

I wanna say MAF. Sensitive little part right there.