When doing your timing belt you adjust your tensioner to get the belt nice and tight or loose for take off.
Well, on the Zetecs, there is a little window with a marker for the optimal spot for your timing belt to be tensioned. As you tightened the belt you will see the marker go to the window. When you turn on your car this shakes a bit and moves some on acceleration, but in idle, it should stay pretty close to the marker in the window.
You will be surprised how that little detail can make a difference.
Did you crank the engine by hand a couple of times and re-checked the timing was still in time after hand cranks?