Feels like it may be the belts slipping. As you shift into each gear, it feels like car is struggling then its like being shot out of a sling shot as it releases. After awhile of driving, the car acts like there is nothing wrong with it. I have had this car since 2003 and it has never done this before. The beginning of this year I had a valve job done and every since it has been stumbling, and now it is getting worse and not safe to drive. Can you please give me some advice on what it may be.