I have a 2001 Dodge Intrepid with a 2.7L engine, just over 100,000 miles. When revving the engine or accelerating to go up an incline the engine momentarily bogs out at just under 3000 RPM. The engine recovers and starts to accelerate again only to bog out again. The timing belt on this car has not been replaced. Could the belt slip or would it completely go if it malfunctioned? The spark plugs have been changed, but gas lines or fuel pump have not been checked. What might be the most likely problem?