I'd suspect three possibilities:
Dirty fuel injectors which can be fixed by adding a couple of cans of
fuel injector cleaner to the fuel.
An engine manifold vacuum leak that allows a small quantity of air to
dilute the air / fuel mixture at idle causing the engine to run lean -
then, when you open the throttle, it runs too lean and stumbles until
the fuel injection catches up to the condition. The leak isn't noticed
above idle because it's small but constant and so becomes a smaller and
smaller % of the total mixture as engine speed rises. Such a small leak
can be found at idle by carefully applying some propane or carburetor
cleaner to suspected sites like the fuel injector seals, intake manifold
gaskets and vacuum hoses attached to the intake manifold.
The engine's ignition "vacuum advance" is sticking. I put it in quotes
because this modern engine probably no longer has a "vacuum advance" as
in the bad old days of carburetors and ignition points etc., but it must
have SOMETHING like it to advance the spark for acceleration and as the