1997 Buick Riviera Tune Up

Your plugs are due for a changed. But I'd go with iridium this time around. Also change the wires/boots, fuel and air filters. And have you ever changed the O2 sensors? If not, they're due for a change. Even when the computer doesn't detect a problem with them, they still fall slightly out of range as they age, and the computer compensates for it, but at the cost of fuel economy ans sometimes overall performance.

With so many miles, it possible other engine components/sensors may need changing. But don't fix what ain't broke.

If your engine continues to run sluggish after the tune up, have the computer and sensors checked out.