Tune up time! Replace all coils and spark plugs it the BEST way to go. Some injector cleaner with Chevron Techron will help also. Now you might get by with a new coil on # 5 cyl but also might have to do that 7 more times. So replacement of ALL coils and plugs is your best bet. A little pricey now but worth every dime in the long run, IF you plan to keep this vehicle!