I have a 02 Jeep Grand Cherokee wj 4.0 motor. so I was running my jeep all day with no problem I get home parked it for like an hour. I go back out I start the jeep sitting in my driveway then it cuts off on me try restarting it nothing so I let it sit for like 2mins tried again it turns on so I leave get down the street and cuts off on me again I did the same thing I did b4 let sit but this time when i started it. was running rough so the check engine light came on I put my scanner on and it has a code p1391 interm lost of cmp or ckp has anyone had this problem before was it easy to replace and how much did it costed I'm thinking buying it from the dealership