Just had the spark plugs replaces as well as the A/C condenser. "Check engine" light came on and mechanic found a loose plug as the code indicated a problem with #4 cylinder. Engine revs past 4000 rpm when vehicle is in "park" but not when accelerating into traffic. Otherwise it runs very smooth but it only gets past 60 mph when you do it slowly rather than trying to merge into freeway traffic or passing.