95 Cutlass Supreme 3100 runs rough at speed

I've got a 1995 Cutlass Supreme with the 3100 engine. When I am at speed, and the engine is running between about 2200 and 2800, it tends to "stutter" a bit, and the tach bounces a bit, especially when it needs a little more power (i.e., going up an incline). It doesn't always happen, only occasionally. Is it an o2 sensor? Where are they located on this vehicle and how hard are they to replace?

The car just had a new alternator put in, headgasket replacement (along with plugs, wires, and a power steering pump) a few months ago, and a rebuilt transmission put in 2 years ago. A little over 200,000 miles on it now.

Any help would be appreciated.

no one has any ideas? Come on, I need some help.