I own a 2005 Stratus with a 2.7 L engine. It has 157,000 miles on it. My daily commute is 45 miles one way. When the car reaches the normal operating temp it starts to buck. This will happen for the next 10 to 20 miles and then will stop. Sometimes it is a quick buck and sometimes it will slow down for 2 or 3 scconds and then take off again. It might happen once or up to a dozen times. It's never the same. The check engine light is not on. I changed the plugs when this all started. Help.