1999 Nissan Pathfinder knock valve?

I just had the timing belt, tune up, oil change and front axel changed on my car. Everything has been great but all of a sudden yesterday when I stop at a light the rpm start fluctuating and then when I try to accelarate it turns off. My son, husband, etc thought that a vaccum hose had come lose but we can't find one. We got it to advanced auto and they put it on machine and said that it is reading a knock sensor. They thought that it should be acting up even when driving but it is not. Do you have any suggestions. Also if I'm in park and sitting the car is fine it is only when I put it in drive and start off.