I have a 2000 A4tqms. I was driving onto an on-ramp the other day and when I shifted to 4th gear the pedal pressure changed suddenly (pedal felt soft and spongy) and the clutch started to slip. I was not accelerating hard (just hard enough to merge at 100-110km/h. The clutch was not slipping before. Now the clutch slips easily if you accelerate too quickly and I can not feel the clutch pick-up point through the pedal any more.

If it was the clutch going wouldn't I feel it start to gradually slip more and more often. I was thinking it could be the slave or master cylinder but all the failures I have read about make it impossible to shift and the pedal does return it just feels lighter and springy. Whith the car off I pushed the clutch pedal in with my hand and I hear creaking from under the car or the engine bay.

The car is always parked indoors and the garage floor is dry, so no leaks there. In the brake fluid resivoir it does look like there were some bubbles in it but the brakes are working perfectly.

Could this be a problem with the pressure plate? I would think it would be making terrible noises if that had failed but there is no new or louder noises from the transmission.

I think the 1.8t has a self adjusting clutch so there isn't anything there to fix... right?

Any suggestions on where to begin with this?

I am not sure if it is relevant but Audi replaced the engine two years ago (20,000km ago) and it has made a clicking noice accelerating in first gear ever since. You can only hear it with the windows down. It is still making the noise but is has never gotten worse.