1994 Buick Century Cause of misfiring, running rough, loss

Engine Performance problem
1994 Buick Century V6 Front Wheel Drive Automatic 138000 miles

Changed the plugs, and fairly sure all wires are where the should be. Feels like its not firing on all cylinders (kind of like me) Ran much smoother before new plugs installed. Rear wires are very difficult to get to and make good there is proper connection. Need to know what to check.