Most common cause of the symptoms you describe is either worn engine bearings. With the high mileage you show, my first thought is; the motor is just plain getting tired. You can double check your dash gauge by removing the OP sending unit, and screwing a regular needle type liquid gauge in it's place but, you will find close to the same pressure reading. Hope this helps.
Tuesday, March 17th, 2009 AT 8:39 PM