My 2005 Dodge neon has 48000 miles on it and has had the spark plugs changed three times. When accelerating on a regular road trying to get to 45 mph I notice that the RPM can climb from 3000 and once in a while 4000 before coming down to 2000 RPM. When it finally hits 45 MPH you can tell the engine bog down and the RPM's fall to 1300 to 1500. Does anyone have any idea why this would be?