The brief
The Touran had an intermittent loss of power under acceleration, the engine going flat for a moment and then picking back up, with no obvious pattern to it. That on-off behaviour is a clue: a part that's failing intermittently rather than one that's failed outright. A power drop that comes and goes like that often traces to an ignition coil. Each cylinder has a coil that fires its spark plug, and when one starts breaking down, it can drop its spark under load, so that cylinder misses and the engine goes flat for a beat. Because it doesn't fail every time, the symptom is intermittent, which is exactly what makes it frustrating. A coil doing that has had its run, and it needs identifying and changing.
The diagnosis
A diagnostic scan and a swap-test of the coils between cylinders pinned it: one coil was breaking down under load, and the misfire followed it when it was moved. The plugs and the rest of the ignition side checked out, so it was that one coil at fault, not the set. That's a targeted replacement, the failed coil, not a shotgun job on the lot.
The work
The faulty ignition coil was unplugged and removed, and a new genuine Volkswagen coil fitted in its place, the harness clip clicked back home. Then the stored fault codes were cleared and the engine run to confirm it was firing cleanly on all cylinders with no miss. A road test confirmed the power and the response were back, with no more flat spots under acceleration.
The outcome
Full power and responsiveness back, no intermittent flat spots, a steady idle, and no fault codes. The Touran went home running cleanly. An intermittently failing coil is the kind of thing that's easy to live with until it gets worse, so identifying the bad one and fitting a genuine replacement put the running properly right.