Q:
Why did you replace the front O2 sensor? Did you have a code indicator and what was it?
On my '97 I changed out both O2 sensors and then found I had a vacuum leak around the intake manifold caused by the gasket; spray some carb cleaner around the intake manifold and see if the engine smooths out or changes rpm. If you have a vacuum leak, try tightening the manifold bolts first and see if that will clear up the problem; otherwise replace the gasket...a real PITA.