Coupling issues (intermittent)

Hey folks

Do you have any further questions about my clutch problems?

Basis:
Car: Beetle
Motor: ALH
Performance: 90 hp engine without power increase
Kilometer: 76000
Portable heater

To the Problem:

The car can be operated without problems for about 10 to 15 minutes, with no issues in the transmission. However, the clutch then fails for a short period. That is, when the turbo engages, the RPMs increase sharply to around 2600. The acceleration then fails, and the Beetle becomes even faster, but not as smoothly as usual. If you then gently press the pedal with your foot for 15 minutes, the situation improves to the point where you can once again drive normally and cheerfully.
I also visited the friendly mechanic today. The only thing they noticed was that the needle on the tachometer 'jitters' when the engine is at full throttle (below 2000 RPM). D.h. a short overswing, then a underswing. (So, apply gas and then hold it).
The stupid thing is that the error, of course, didn't occur at the car dealership.
Furthermore, at the car dealership, the opinion is that once the clutch has failed, it will only slip and now they are blaming the electronics.
They also tried to read the error codes, but I could have told them there was nothing there.
The problem is also that my warranty expired on 15.12.2003, and when I asked about any possible goodwill considering that I will likely need a new clutch soon, the response was simply 'not much.'
How long should a clutch last? I've never had to replace a clutch in any of my previous cars.