In your post I see no mention of having changed the fuel filter. I am sure the dealership mechanic would have done that. One thing to consider is that the fuel filter may be clogged again. I once worked on a 280Z that had drooping fuel pump pressure. Changing the fuel filter would cure the problem but the stumbling and loss of power would return. After the fuel tank was steam cleaned and a new filter installed, the problem went away; didn’t come back; and the Z ran like a champ.
Hope your solution is the same. Let us know the final solution to this problem.