97 Grand Prix Sputtering

I just started having some issues with my 97 Grand Prix. Lately it has started to feel like it is sputtering or not getting enough gas. It does it around 25 - 35 mph, and when going up hills. It tends to do it more often when the car is warm.

I had a mechanic test drive the car, who said it seemed fine, however, I felt it sputter while he was driving it.

What could this be?!? Please help! :banghead:

Any check engine light? When was your last tune up?

My last tune up was probably about a year ago. No check engine light.