What could be causing the engine to jump timing when you shut it off or on first crank before starting?

1997 LHS 3.5L 165K. Had waterpump changed. They were supposed to do timing also but didn't. Car ran 3 days fine, then wouldn't start. Had exhaust backpressure sound like it had jumped time. A different Mech changed crank sensor and said timing was off three teeth. Had them "do the timing" they replaced the belt but not pulley or tensioner. Car ran 3 days and then would not start. I changed the cam sensor with no results so had it towed to the garage. This time they changed the tensioner and pulley. They said they drove it 3 different times and it was good to go. When I went to pick it up it wouldn't start again, like it had jumped time again. Luckily it's only doing it either when you shut the car off or on the first crank before it starts. What could be causing this timing jump/ no start issue. Trying to help the mech. And I need my car. Please help! It's already cost $1100 and the car still isn't running.