97 3.8 cougar bad prob

97 cougar 3.8 v6 check engine light on codes are p0305 and p0306.missfire cylinder#5 and#6 runs like crap idles like crap and whene checkengine light blinks runs even wourse like it might stall.if i turn the car off and let it sit for about 5seconds it runs ok but starts doing it again after driving for a while.replaced enjector on cylinder#4 and it ran great for about an hour the ce light came back on and it did the same thing it was doing the day before.if anyone can help please do so or i might set my cougar on fire.thankyou.