Audi A3 1999: Motor houdt in.

Excuse my English - I read Dutch but cannot write yet.

When accelerating, there are sudden hick-ups, like the loosing power for 0.2 seconds, and then it starts to accelerate.
Also when the motor is cold, and one accelerate cautiously below 2000rpm, there are sometimes very brief moments when the motor feels like cutting off for 0.2 seconds.
When cold and driving very cautiously, 1200-1500rpm, it feels like the motor is resisting (motorbreaking).
When parked, and revving the engine very cautiously, there are some light hesitations around 1800-2200 rpm.

When accelerating heavily, it accelerates very well till 4000 RPM thereafter it stops accelerating. Only when you release a litle bit the gas-pedal, it starts again acceleration.

I have changed:
1) spark plugs (4)
2) coils (4)
3) mass air meter.
4) fuel filter.
5) air filter.

Looked for vacuum tubes, but not identified any leaks.

The problem is still there, maybe not so badly, but is still there. Do I need to change the vacuum tubes or what is the problem?

Ik kan Nederlands lezen, dus uw antwoorden in het Nederlands zijn zeer gewaardeerd.