Sputtering problem with 2002 mustang

[Jr]

Hey guys,
What would cause a 2002 ford mustang to sputter at 60-65 MPH? The wife's car just started doing this weird sputter on the freeway. What's funny, the RPM to don't fluctuate, the speed of the car stays the same, and power stays on the whole time the car is in motion. This is weird.

Breakdown of the car:
Ford
Mustang
auto
3.6 or 3.8 V6
everything is stock on this car.

We would appreciate any help on the subject.