I just purchased this 97 DeVille, was running fine, January 2014 had head casket repair. Now showing P0340 code. Replaced CMP sensor, still rough on idle. Took to local shop and they are telling me that it is the timing and you have to spend 12 hours just to get timing right. Does this sound right? Should I replace CKP sensors first? Help, I can't afford to spend much on this.